ci: GitLab CI is broken (autoreconf)

[kmk3]

GitLab CI has been broken since at least the pipeline of commit 836ffe3
("compile fix", 2022-08-29):

• https://gitlab.com/Firejail/firejail_ci/-/pipelines/625517735

Partial error log from the debian_ci job at
https://gitlab.com/Firejail/firejail_ci/-/jobs/2947880865:

make[1]: Leaving directory '/builds/Firejail/firejail_ci'
   dh_clean
 dpkg-source -i -I -b .
dpkg-source: warning: upstream signing key but no upstream tarball signature
dpkg-source: info: using source format '3.0 (quilt)'
dpkg-source: info: building firejail using existing ./firejail_0.9.71.orig.tar.gz
dpkg-source: info: building firejail in firejail_0.9.71-0.1~ci.debian.tar.xz
dpkg-source: info: building firejail in firejail_0.9.71-0.1~ci.dsc
 debian/rules binary
dh binary
   dh_update_autotools_config
   dh_autoreconf
configure.ac:42: error: possibly undefined macro: AS_HELP_STRING
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
autoreconf: error: /usr/bin/autoconf failed with exit status: 1
dh_autoreconf: error: autoreconf -f -i returned exit code 1
make: *** [debian/rules:8: binary] Error 25
dpkg-buildpackage: error: debian/rules binary subprocess returned exit status 2
debuild: fatal error at line 1182:
dpkg-buildpackage -us -uc -ui -i -I failed
gbp:error: 'debuild -i -I -uc -us' failed: it exited with 29
Cleaning up project directory and file based variables 00:01
ERROR: Job failed: exit code 1

Likely caused by commit 54cb3e7 ("Merge pull request #5315 from
ChrysoliteAzalea/landlock", 2022-08-29).

Should be fixed by #5347.

Cc: @netblue30 @reinerh

[reinerh]

There was a small syntax error in configure.ac. I pushed a fix.