[face] Remove const from get_user_data prototype

This was done by mistake.

Since the returned user_data can be changed, face should not be marked
const. Other object types follow this parttern.

src/hb-face.cc

@@ -342,7 +342,7 @@ hb_face_set_user_data (hb_face_t          *face,
  * Since: 0.9.2
  **/
 void *
-hb_face_get_user_data (const hb_face_t    *face,
+hb_face_get_user_data (hb_face_t          *face,
 		       hb_user_data_key_t *key)
 {
   return hb_object_get_user_data (face, key);

src/hb-face.h

@@ -96,7 +96,7 @@ hb_face_set_user_data (hb_face_t          *face,
 		       hb_bool_t           replace);
 
 HB_EXTERN void *
-hb_face_get_user_data (const hb_face_t    *face,
+hb_face_get_user_data (hb_face_t          *face,
 		       hb_user_data_key_t *key);
 
 HB_EXTERN void