Fix wx/display.h compilation when it's the first included header

Include wx/defs.h to ensure that wxUSE_DISPLAY is defined before it is
checked for.

include/wx/display.h

@@ -10,6 +10,8 @@
 #ifndef _WX_DISPLAY_H_BASE_
 #define _WX_DISPLAY_H_BASE_
 
+#include "wx/defs.h"
+
 // NB: no #if wxUSE_DISPLAY here, the display geometry part of this class (but
 //     not the video mode stuff) is always available but if wxUSE_DISPLAY == 0
 //     it becomes just a trivial wrapper around the old wxDisplayXXX() functions