Make hb_language_t typesafe

src/hb-common.h

diff --git a/src/hb-common.h b/src/hb-common.h
index 606e117..4dadc94 100644
--- a/src/hb-common.h
+++ b/src/hb-common.h
@@ -99,7 +99,7 @@
 
 /* hb_language_t */
 
-typedef const void *hb_language_t;
+typedef struct _hb_language_t *hb_language_t;
 
 hb_language_t
 hb_language_from_string (const char *str);