gtaskqueue: Fix a typo

This is a no-op in practice since gtaskqueue_thread_enqueue() and
taskqueue_thread_enqueue() are identical, and while _gtaskqueue_create()
compares the enqueue callback pointer with gtaskqueue_thread_enqueue(),
the result has no effect since TQ_FLAGS_UNLOCKED_ENQUEUE was copied
directly from subr_taskqueue.c and is unused in the gtaskqueue code.

Fix it anyway since it's a bug. More generally we really need to
consolidate subr_taskqueue.c and subr_gtaskqueue.c.

PR: 270661
MFC after: 1 week

(cherry picked from commit e1f336704d308679d4178fd5063dba854b22a07b)