WPrefs: Move declaration of function into the common header

It is a bad idea to declare the prototype of an external function
in a file as it won't allow the compiler to cross-check it.

WPrefs.app/MouseSettings.c

@@ -95,7 +95,6 @@ static char *wheelActions[2];
 
 #define DELAY(i)		((i)*75+170)
 
-int ModifierFromKey(Display * dpy, char *key);
 
 static void setMouseAccel(WMScreen * scr, float accel, int threshold)
 {

WPrefs.app/WPrefs.h

@@ -115,6 +115,10 @@ void SetSpeedForKey(int speed, const char *defaultName);
 void AddDeadChildHandler(pid_t pid, void (*handler)(void*), void *data);
 
 
+/* ---[ xmodifier.c ] ---------------------------------------------------- */
+int ModifierFromKey(Display * dpy, char *key);
+
+
 #define FRAME_TOP	105
 #define FRAME_LEFT	-2
 #define FRAME_WIDTH	524