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/MouseSettings.c

@@ -158,7 +158,6 @@ static void doubleClick(WMWidget * w, void *data)
 {
 	_Panel *panel = (_Panel *) data;
 	int i;
-	extern _WINGsConfiguration WINGsConfiguration;
 	char buffer[32];
 
 	for (i = 0; i < 5; i++) {