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.

src/icon.c

@@ -416,7 +416,8 @@ char *get_name_for_instance_class(char *wm_instance, char *wm_class)
 
 static char *get_icon_cache_path(void)
 {
-	char *prefix, *path;
+	const char *prefix;
+	char *path;
 	int len, ret;
 
 	prefix = wusergnusteppath();