FFI: Turn FFI finalizer table into a proper GC root.

Reported by Sergey Bronnikov. #1168

src/lj_obj.h

@@ -579,6 +579,9 @@ typedef enum {
   GCROOT_BASEMT_NUM = GCROOT_BASEMT + ~LJ_TNUMX,
   GCROOT_IO_INPUT,	/* Userdata for default I/O input file. */
   GCROOT_IO_OUTPUT,	/* Userdata for default I/O output file. */
+#if LJ_HASFFI
+  GCROOT_FFI_FIN,	/* FFI finalizer table. */
+#endif
   GCROOT_MAX
 } GCRootID;