wmaker: Replaced local 'extern' definition by proper header include

It is dangerous to define an extern variable locally because the
compiler is not able to ensure proper type definition, but including
the header that defines it allows a safer situation.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

src/xmodifier.c

@@ -36,9 +36,9 @@ Perpetrator: Sudish Joseph <sj@eng.mindspring.net>, Sept. 1997. */
 #include <X11/XKBlib.h>
 
 #include <WINGs/WUtil.h>
+#include "WindowMaker.h"
 #include "xmodifier.h"
 
-extern Display *dpy;
 
 /************************************************************************/
 /*                            keymap handling                           */