build: Add missing include

util-private.h includes glib-i18n-lib.h, which requires GETTEXT_PACKAGE
to be defined. The define comes from config.h,
but that cannot be included in headers, so we have to make sure
that any source file that pulls in util-private.h (or a header
that includes it) includes config.h first.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2566>

src/meta-private-enum-types.c.in

@@ -1,4 +1,5 @@
 /*** BEGIN file-header ***/
+#include "config.h"
 #include "meta-private-enum-types.h"
 /*** END file-header ***/