fix gcc warnings

Max Kellermann <max@duempel.org>
diff --git a/extensions/dscp_helper.c b/extensions/dscp_helper.c
index 8b60d41..9e9a163 100644
--- a/extensions/dscp_helper.c
+++ b/extensions/dscp_helper.c
@@ -49,7 +49,7 @@
 static unsigned int
 class_to_dscp(const char *name)
 {
-	int i;
+	unsigned i;
 
 	for (i = 0; i < sizeof(ds_classes) / sizeof(struct ds_class); i++) {
 		if (!strncasecmp(name, ds_classes[i].name,