Mark unsigned integer literals with the u suffix

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.
diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 5f4cd42..046df97 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -626,7 +626,7 @@
 
 /* Script/language-system/feature index */
 struct Index : USHORT {
-  static const unsigned int NOT_FOUND_INDEX = 0xFFFF;
+  static const unsigned int NOT_FOUND_INDEX = 0xFFFFu;
 };
 DEFINE_NULL_DATA (Index, "\xff\xff");