gdb_monitor: build error with -std=gnu++20

[cproc]

When building app/gdb_monitor with -std=gnu++20 as default, the following error occurs:

In file included from /.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbsupport/
def-vector.h:22,
                 from /.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbsupport/
byte-vector.h:21,
                 from /.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbsupport/
common-utils.h:25:
/.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbsupport/default-init-alloc.h: 
In instantiation of 'class gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> >':
/.../contrib/stdcxx-4eddc2a55a80ed5d3a50fee3f5c25e7ac42afd72/include/stdcxx/bits/stl_vector.h:88:21:   requi
red from 'struct std::_Vector_base<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > >'
/.../contrib/stdcxx-4eddc2a55a80ed5d3a50fee3f5c25e7ac42afd72/include/stdcxx/bits/stl_vector.h:423:11:   requ
ired from 'class std::vector<unsigned char, gdb::default_init_allocator<unsigned char, std::allocator<unsigned char> > >'
/.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbserver/server.cc:571:3:   requ
ired from here
/.../contrib/gdb-59a56418c6720518cfc7e4485557dd2c707b5e03/src/noux-pkg/gdb/gdbsupport/default-init-alloc.h:52:12: error: 'construct' has not been declared in 'class std::allocator<unsigned char>'
   52 |   using A::construct;
      |            ^~~~~~~~~