Always include X11 headers from the private X11 display header

Commit 28ab24cd55 excluded too much in
wxUSE_DISPLAY==0 case, we still need X11 headers when not using
wxUSE_DISPLAY.

include/wx/unix/private/displayx11.h

@@ -12,14 +12,14 @@
 
 #include "wx/defs.h"
 
+#include <X11/Xlib.h>
+#include <X11/Xatom.h>
+
 #if wxUSE_DISPLAY
 
 #include "wx/log.h"
 #include "wx/translation.h"
 
-#include <X11/Xlib.h>
-#include <X11/Xatom.h>
-
 #ifdef HAVE_X11_EXTENSIONS_XF86VMODE_H
 
 #include <X11/extensions/xf86vmode.h>