build: Only install group.h header when built with x11

Also rename the header to file to be consistent with other x11-only
headers

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3776>

src/core/window.c

@@ -83,10 +83,10 @@
 #include "meta/prefs.h"
 
 #ifdef HAVE_X11_CLIENT
-#include "meta/group.h"
 #include "mtk/mtk-x11.h"
 #include "x11/meta-x11-display-private.h"
 #include "x11/meta-x11-frame.h"
+#include "x11/meta-x11-group-private.h"
 #include "x11/window-props.h"
 #include "x11/window-x11-private.h"
 #include "x11/window-x11.h"