warnings when building lockstep plugin on s390

Near the top of a build log for the ubuntu-20.04-s390x-all CI job are the following warnings for the lockstep plugin:

(example job: https://gitlab.com/qemu-project/qemu/-/jobs/6138710238 -- you'll need to follow the "Complete Raw" link to see this bit of the output):

/home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/contrib/plugins/lockstep.c: In function ‘report_divergance’:
/home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/contrib/plugins/lockstep.c:133:38: warning: implicit declaration of function ‘g_memdup2’; did you mean ‘g_memdup’? [-Wimplicit-function-declaration]
  133 |                                      g_memdup2(&divrec, sizeof(divrec)));
      |                                      ^~~~~~~~~
      |                                      g_memdup
/home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/contrib/plugins/lockstep.c:133:38: warning: passing argument 2 of ‘g_slist_prepend’ makes pointer from integer without a cast [-Wint-conversion]
  133 |                                      g_memdup2(&divrec, sizeof(divrec)));
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                      |
      |                                      int
In file included from /usr/include/glib-2.0/glib/gmain.h:26,
                 from /usr/include/glib-2.0/glib/giochannel.h:33,
                 from /usr/include/glib-2.0/glib.h:54,
                 from /home/gitlab-runner/builds/-LCfcJ2T/0/qemu-project/qemu/contrib/plugins/lockstep.c:26:
/usr/include/glib-2.0/glib/gslist.h:61:10: note: expected ‘gpointer’ {aka ‘void *’} but argument is of type ‘int’
   61 | GSList*  g_slist_prepend                 (GSList           *list,
      |          ^~~~~~~~~~~~~~~

Two problems here:

1. Warnings should make the build fail, so something somewhere here has not got the right set of warning/-Werror flags
2. We shouldn't be generating a warning. g_memdup2() only exists on glib 2.68 or newer, which is why our include/glib-compat.h has a back-compat implementation for it. Since plugins don't get to use our back-compat machinery, they should avoid g_memdup2() (or provide their own back-compat handling).

Bug introduced in commit ee293103 ("plugins: update lockstep to use g_memdup2") by @stsquad :-)