grefcount: add missing gatomic.h
grefcount: add missing gatomic.h

Without gatomic.h, build fails on:
In file included from garcbox.c:24:0:
garcbox.c: In function ‘g_atomic_rc_box_acquire’:
grefcount.h:101:13: error: implicit declaration of function ‘g_atomic_int_get’; did you mean ‘__atomic_store’? [-Werror=implicit-function-declaration]
     (void) (g_atomic_int_get (rc) == G_MAXINT ? 0 : g_atomic_int_inc ((rc))); \
             ^
garcbox.c:292:3: note: in expansion of macro ‘g_atomic_ref_count_inc’
   g_atomic_ref_count_inc (&real_box->ref_count);

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib