Use libX11's Xkb* API unconditionally

At this point there shouldn't be any system capable of running mutter
that doesn't have it and we're introducing functionality like setting
the keymap that has an hard requirement on it.

https://bugzilla.gnome.org/show_bug.cgi?id=734301

src/core/bell.h

@@ -18,13 +18,10 @@
  */
 
 #include <X11/Xlib.h>
-#ifdef HAVE_XKB
 #include <X11/XKBlib.h>
-#endif
 #include "display-private.h"
 #include "frame.h"
 
-#ifdef HAVE_XKB
 /**
  * meta_bell_notify:
  * @display: The display the bell event came in on
@@ -37,7 +34,6 @@
  * If the configure script found we had no XKB, this does not exist.
  */
 void meta_bell_notify (MetaDisplay *display, XkbAnyEvent *xkb_ev);
-#endif
 
 /**
  * meta_bell_set_audible: