BUILD/MINOR: ssl: avoid a build warning on LIBRESSL_VERSION with -Wundef

Openssl-compat emits a warning for the test on LIBRESSL_VERSION that might
be underfined, if built with -Wundef. The fix is easy, let's do it. Related
to GH issue #1369.

(cherry picked from commit 6e5542e9f45bd72ef3975f1dc4d4193a824f5e03)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed