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-buffer-serialize.cc b/src/hb-buffer-serialize.cc
index 263bc81..406d69d 100644
--- a/src/hb-buffer-serialize.cc
+++ b/src/hb-buffer-serialize.cc
@@ -63,7 +63,7 @@
 hb_buffer_serialize_format_from_string (const char *str, int len)
 {
   /* Upper-case it. */
-  return (hb_buffer_serialize_format_t) (hb_tag_from_string (str, len) & ~0x20202020);
+  return (hb_buffer_serialize_format_t) (hb_tag_from_string (str, len) & ~0x20202020u);
 }
 
 /**