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.

WINGs/userdefaults.c

@@ -83,7 +83,7 @@ const char *wusergnusteppath()
 char *wdefaultspathfordomain(const char *domain)
 {
 	char *path;
-	char *gspath;
+	const char *gspath;
 	size_t slen;
 
 	gspath = wusergnusteppath();