selftests/tls: fix selftest with CHACHA20-POLY1305

TLS selftests were broken also because of use of structure that
was not exported to UAPI. Fix by defining the union in tests.

Fixes: 4f336e88 (selftests/tls: add CHACHA20-POLY1305 to tls selftests)
Reported-by: Rong Chen <rong.a.chen@intel.com>
Signed-off-by: Vadim Fedorenko <vfedorenko@novek.ru>
Link: https://lore.kernel.org/r/1612384634-5377-1-git-send-email-vfedorenko@novek.ru


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