WPrefs: Cleaned dangerous function prototype usage

- remove extern declaration in source file, use header instead

- add inclusion of header defining the functions of the file to
get the compiler to cross-check them

- marked static the functions that should not be visible ouside
their file

WPrefs.app/xmodifier.c

@@ -39,6 +39,8 @@ Perpetrator: Sudish Joseph <sj@eng.mindspring.net>, Sept. 1997. */
 
 #include <WINGs/WUtil.h>
 
+#include "WPrefs.h"
+
 /************************************************************************/
 /*                            keymap handling                           */
 /************************************************************************/