Enable mutex witnesses even when !isthreaded.
Enable mutex witnesses even when !isthreaded.

This fixes interactions with witness_assert_depth[_to_rank](), which was
added in dad74bd3c811ca2b1af1fd57b28f2456da5ba08b (Convert
witness_assert_lockless() to witness_assert_lock_depth().).

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc