Drop use of volatile

As detailed in
gitlab.gnome.org/GNOME/glib/-/issues/600#note_877282, volatile
isn't actually needed in these contexts because the atomic operations
already give us strong enough guarantees. In GCC 11, this triggers a
diagnostic due to the volatile qualifier getting dropped anyway.

There is a WIP to do the same in glib:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

This obsoletes this downstream patch:
https://src.fedoraproject.org/rpms/ostree/c/b8c5a6fb

src/libostree/ostree-enumtypes.c.template

@@ -36,9 +36,9 @@
 GType
 _@enum_name@_get_type (void)
 {
-  static volatile gsize the_type__volatile = 0;
+  static gsize static_the_type = 0;
 
-  if (g_once_init_enter (&the_type__volatile))
+  if (g_once_init_enter (&static_the_type))
     {
       static const G@Type@Value values[] = {
 /*** END value-header ***/
@@ -57,10 +57,10 @@ _@enum_name@_get_type (void)
         g_intern_static_string ("@EnumName@"),
         values);
 
-      g_once_init_leave (&the_type__volatile, the_type);
+      g_once_init_leave (&static_the_type, the_type);
     }
 
-  return the_type__volatile;
+  return static_the_type;
 }
 
 /*** END value-tail ***/