tls-crypt: avoid warnings when --disable-crypto is used

tls-crypt: avoid warnings when --disable-crypto is used

Avoid including the content of tls_crypt.h when --disable-crypto
is used, as it will trigger some warnings due to missing
structures declarations.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170707102238.8781-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15014.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 2dfbf62b6ace1eb39f1ae7126bc5530a541bed58)