[shape] Use hb_memcmp instead of memcmp

src/hb-ot-shape.hh

@@ -51,7 +51,7 @@ struct hb_ot_shape_plan_key_t
 
   bool equal (const hb_ot_shape_plan_key_t *other)
   {
-    return 0 == memcmp (this, other, sizeof (*this));
+    return 0 == hb_memcmp (this, other, sizeof (*this));
   }
 };