configure.ac: add check for getegid function

We already use this function and so far we've been lucky that the same
check is done by gnulib.  This will change once we will drop gnulib and
also make it obvious that we have to do the same check in Meson as well.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

configure.ac

@@ -354,6 +354,7 @@ dnl Availability of various common functions (non-fatal if missing),
 dnl and various less common threadsafe functions
 AC_CHECK_FUNCS_ONCE([\
   fallocate \
+  getegid \
   geteuid \
   getgid \
   getifaddrs \