Otherwise two consequative submits may have the same
num_busy_workers value if threadpool_worker_proc did not
have a chance to pick the work in between. A new thread is
not created in tp_submit() for the second submit and if the
first submit queues an (infinitely long) wait
the next submitted callback is not called for indefinite
time period.

Fixes hang on exit in Detroit Become Human.
Signed-off-by: Paul Gofman <pgofman@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>