cleanup: Remove not used X11 includes

Also add ones where they are actually used

Helps https://gitlab.gnome.org/GNOME/mutter/-/issues/2272

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3603>

src/backends/meta-idle-monitor.c

@@ -29,8 +29,6 @@
 #include "config.h"
 
 #include <string.h>
-#include <X11/Xlib.h>
-#include <X11/extensions/sync.h>
 
 #include "backends/gsm-inhibitor-flag.h"
 #include "backends/meta-backend-private.h"

src/backends/x11/meta-backend-x11.h

@@ -24,6 +24,8 @@
 
 #include <stdint.h>
 #include <X11/Xlib.h>
+#include <X11/extensions/XInput.h>
+#include <X11/extensions/XInput2.h>
 
 #include "backends/meta-backend-private.h"
 #include "backends/x11/meta-backend-x11-types.h"