streams: openssl: fix bogus warning on unused parameter

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)`.

src/streams/openssl.c

@@ -252,7 +252,8 @@ int git_openssl_stream_global_init(void)
 #if defined(GIT_THREADS)
 static void threadid_cb(CRYPTO_THREADID *threadid)
 {
-    CRYPTO_THREADID_set_numeric(threadid, git_thread_currentid());
+	GIT_UNUSED(threadid);
+	CRYPTO_THREADID_set_numeric(threadid, git_thread_currentid());
 }
 #endif