gateway: Fix leaking uuid

==1612==    by 0x4E80975: g_try_malloc0 (in /usr/lib/libglib-2.0.so.0.3600.3)
==1612==    by 0x17D947: bt_uuid2string (glib-helper.c:76)
==1612==    by 0x184A84: adapter_emit_uuids_updated.part.8 (adapter.c:1086)
==1612==    by 0x129A11: update_adapter_uuid_ready (gateway.c:718)
==1612==    by 0x12AADA: gateway_init (gateway.c:882)
==1612==    by 0x128160: handle_uuid (manager.c:199)
==1612==    by 0x4E97807: g_slist_foreach (in /usr/lib/libglib-2.0.so.0.3600.3)
==1612==    by 0x12906A: audio_probe (manager.c:855)
==1612==    by 0x18C681: device_probe_drivers (device.c:1388)
==1612==    by 0x18CD62: btd_device_add_uuid (device.c:3041)
==1612==    by 0x12920E: hf_io_cb (manager.c:650)