Add hb_language_t as well as language/script conversion to OT tags

src/hb-ot.h

diff --git a/src/hb-ot.h b/src/hb-ot.h
index c04216b..eab888e 100644
--- a/src/hb-ot.h
+++ b/src/hb-ot.h
@@ -30,5 +30,6 @@
 #include "hb.h"
 
 #include "hb-ot-layout.h"
+#include "hb-ot-tag.h"
 
 #endif /* HB_OT_H */