FIXUP: gatt: Add listen socket for GATT server

Fix these file descriptor leaks:

==858== Open pf-31 socket 9:
==858==    at 0x42919F7: socket (socket.S:64)
==858==    by 0x80A8213: bt_io_listen (btio.c:1470)
==858==    by 0x80CE621: btd_gatt_service_manager_init (gatt.c:2182)
==858==    by 0x80A926F: main (main.c:549)
==858==
==858== Open pf-31 socket 8:
==858==    at 0x42919F7: socket (socket.S:64)
==858==    by 0x80A8213: bt_io_listen (btio.c:1470)
==858==    by 0x80CE563: btd_gatt_service_manager_init (gatt.c:2169)
==858==    by 0x80A926F: main (main.c:549)