Re-enable trace stitching.

Thanks to Vyacheslav Egorov.

src/lj_gc.c

@@ -69,7 +69,7 @@ static void gc_mark(global_State *g, GCobj *o)
       gray2black(o);  /* Closed upvalues are never gray. */
   } else if (gct != ~LJ_TSTR && gct != ~LJ_TCDATA) {
     lua_assert(gct == ~LJ_TFUNC || gct == ~LJ_TTAB ||
-	       gct == ~LJ_TTHREAD || gct == ~LJ_TPROTO);
+	       gct == ~LJ_TTHREAD || gct == ~LJ_TPROTO || gct == ~LJ_TTRACE);
     setgcrefr(o->gch.gclist, g->gc.gray);
     setgcref(g->gc.gray, o);
   }