keybindings: Move X11 bits to a separate header

Reduces the noise in terms of ifdef and makes it much easier
to spot which X11 bits are still mixed in the generic
keybindings file

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

src/x11/meta-x11-display.c

@@ -61,6 +61,7 @@
 #include "x11/events.h"
 #include "x11/group-props.h"
 #include "x11/meta-x11-frame.h"
+#include "x11/meta-x11-keybindings-private.h"
 #include "x11/meta-x11-selection-private.h"
 #include "x11/window-props.h"
 #include "x11/window-x11.h"