NFC: Export target lost function

NFC drivers will call this routine when they detect that a tag leaves the
RF field. This will eventually lead to the corresponding netlink event
to be sent.

Signed-off-by: Eric Lapuyade <eric.lapuyade@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>