Our provided callback function `threadid_cb(CRYPTO_THREADID
*threadid)` sets up a unique thread ID by asking pthread for the
current thread ID.  Since openssl version 1.1,
`CRYPTO_THREADID_set_numeric` is simply a no-op macro, leaving
the `threadid` argument unused after the preprocessor has
processed the macro. GCC does not account for that situation and
will thus complain about `threadid` being unused.

Silence this warning by using `GIT_UNUSED(threadid)`.