glib 2.56's g_steal_pointer() won't tolerate a function pointer in place
of a gpointer.

  CC       src/src_libNetworkManager_la-nm-active-connection.lo
    src/nm-active-connection.c:1017:17: error: pointer type mismatch
      ('NMActiveConnectionAuthResultFunc' (aka 'void (*)(struct _NMActiveConnection *,
      int, const char *, void *)') and 'gpointer' (aka 'void *'))
      [-Werror,-Wpointer-type-mismatch]
                result_func = g_steal_pointer (&priv->auth.result_func);
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/include/glib-2.0/glib/gmem.h:200:6: note: expanded from macro 'g_steal_pointer'
    (0 ? (*(pp)) : (g_steal_pointer) (pp))
       ^ ~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

There's just a single spot we use it that way, so it's perhaps better to
work around the warning instead of disabling it.