dix: Move DeviceFocusEvent from Xi into enterleave.c

This is only called from the enterleave implementation, so move it and its
helper functions to there. No functional changes.

Fixes build error introduced in 31174565ec if
building with '-Werror=implicit-function-declaration'

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>

include/exevents.h

@@ -162,13 +162,6 @@ extern void
  ProcessOtherEvent(InternalEvent * /* ev */ ,
                    DeviceIntPtr /* other */ );
 
-extern void
- DeviceFocusEvent(DeviceIntPtr /* dev */ ,
-                  int /* type */ ,
-                  int /* mode */ ,
-                  int /* detail */ ,
-                  WindowPtr /* pWin */ );
-
 extern int
  CheckGrabValues(ClientPtr /* client */ ,
                  GrabParameters * /* param */ );