gvfs-gdu-volume-monitor: disable afc to fix build

* sometimes it autodetects libimobiledevice and fails to build:
  | gvfsbackendafc.c: In function '_idevice_event_cb':
  | gvfsbackendafc.c:352:25: error: 'idevice_event_t' has no member named 'uuid'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>