Fixed const correctness in functions using 'wusergnusteppath'

The change introduced in previous commit for const correctness has
a small impact on WindowMaker's code, this patch fixes all the new
warnings.

WPrefs.app/Menu.c

@@ -1478,7 +1478,7 @@ static WMPropList *getDefaultMenu(_Panel * panel)
 
 static void showData(_Panel * panel)
 {
-	char *gspath;
+	const char *gspath;
 	char *menuPath;
 	WMPropList *pmenu;