Fix Ruby 3.1 compatibility

[casperisfine]

Fix: #108

The allocator function need to be explictly defined.

@malmckay

cc @peterzhu2118: would love if you could give this a quick look.

[malmckay]

Looks great, thanks for tracking this down.

[casperisfine]

Welcome. Not too sure what's wrong with the 3.1/macOS CI:

test -z "/Users/runner/work/zookeeper/zookeeper/ext/bin" || /Users/runner/work/zookeeper/zookeeper/ext/zkc-3.4.5/c/install-sh -d "/Users/runner/work/zookeeper/zookeeper/ext/bin"
  /bin/sh ./libtool --mode=install /usr/bin/install -c 'cli_st' '/Users/runner/work/zookeeper/zookeeper/ext/bin/cli_st'
/usr/bin/install -c .libs/cli_st /Users/runner/work/zookeeper/zookeeper/ext/bin/cli_st
  /bin/sh ./libtool --mode=install /usr/bin/install -c 'cli_mt' '/Users/runner/work/zookeeper/zookeeper/ext/bin/cli_mt'
/usr/bin/install -c .libs/cli_mt /Users/runner/work/zookeeper/zookeeper/ext/bin/cli_mt
  /bin/sh ./libtool --mode=install /usr/bin/install -c 'load_gen' '/Users/runner/work/zookeeper/zookeeper/ext/bin/load_gen'
/usr/bin/install -c .libs/load_gen /Users/runner/work/zookeeper/zookeeper/ext/bin/load_gen
test -z "/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper" || /Users/runner/work/zookeeper/zookeeper/ext/zkc-3.4.5/c/install-sh -d "/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper"
 /usr/bin/install -c -m 644 'include/zookeeper.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/zookeeper.h'
 /usr/bin/install -c -m 644 'include/zookeeper_version.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/zookeeper_version.h'
 /usr/bin/install -c -m 644 'include/zookeeper_log.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/zookeeper_log.h'
 /usr/bin/install -c -m 644 'include/proto.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/proto.h'
 /usr/bin/install -c -m 644 'include/recordio.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/recordio.h'
 /usr/bin/install -c -m 644 'generated/zookeeper.jute.h' '/Users/runner/work/zookeeper/zookeeper/ext/include/zookeeper/zookeeper.jute.h'
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
checking for rb_thread_blocking_region()... 
need configuration options.

Provided configuration options:
	--with-opt-dir
	--without-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/Users/runner/hostedtoolcache/Ruby/3.1.2/x64/bin/$(RUBY_BASE_NAME)
/Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:498:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:591:in `try_link0'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:609:in `try_link'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:830:in `try_func'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:1119:in `block in have_func'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:1007:in `block in checking_for'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:362:in `block (2 levels) in postpone'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:332:in `open'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:362:in `block in postpone'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:332:in `open'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:358:in `postpone'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:1006:in `checking_for'
	from /Users/runner/hostedtoolcache/Ruby/3.1.2/x64/lib/ruby/3.1.0/mkmf.rb:1118:in `have_func'
	from extconf.rb:98:in `<main>'
rake aborted!

It works fine on my machine :/