[aat] Ignore GSUB table of Muthu Foundry if they have morx table

Fixes https://github.com/harfbuzz/harfbuzz/issues/1410
diff --git a/src/hb-ot-layout-gpos-table.hh b/src/hb-ot-layout-gpos-table.hh
index 907fd46..2589218 100644
--- a/src/hb-ot-layout-gpos-table.hh
+++ b/src/hb-ot-layout-gpos-table.hh
@@ -1643,6 +1643,9 @@
   inline bool sanitize (hb_sanitize_context_t *c) const
   { return GSUBGPOS::sanitize<PosLookup> (c); }
 
+  HB_INTERNAL bool is_blacklisted (hb_blob_t *blob,
+				   hb_face_t *face) const;
+
   typedef GSUBGPOS::accelerator_t<GPOS> accelerator_t;
 };