selftests/net: enable io_uring sendzc testing

d8b6171b ("selftests/io_uring: test zerocopy send") added io_uring
zerocopy tests but forgot to enable it in make runs. Add missing
io_uring_zerocopy_tx.sh into TEST_PROGS.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/28e743602cdd54ffc49f68bbcbcbafc59ba22dc2.1664142210.git.asml.silence@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>