Fix icon.c unused variable compiler warning

Just to please the compiler as it's reporting:
icon.c: In function 'get_icon_cache_path':
icon.c:425:13: warning: unused variable 'len' [-Wunused-variable]

src/icon.c

@@ -422,7 +422,7 @@ static char *get_icon_cache_path(void)
 {
 	const char *prefix;
 	char *path;
-	int len, ret;
+	int ret;
 
 	prefix = wuserdatapath();
 	path = wstrconcat(prefix, CACHE_ICON_PATH "/");