target: Make __target_execute_cmd() available as extern

Required by COMPARE_AND_WRITE for write instance user-data
submission, in order to bypass target_execute_cmd() checks.

Reported-by: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@purestorage.com>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Nicholas Bellinger <nab@daterainc.com>