Fix unused variable 'libver' warning in Modules/_ssl.c

It can be seen on various buildbots like 3.x.cea-indiana-amd64
and 3.x.murray-snowleopard:

/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Modules/_ssl.c:2227: warning: unused variable 'libver'

/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Modules/_ssl.c:2227: warning: unused variable ‘libver’

Modules/_ssl.c

@@ -2224,7 +2224,9 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
     PySSLContext *self;
     long options;
     SSL_CTX *ctx = NULL;
+#if defined(SSL_MODE_RELEASE_BUFFERS)
     unsigned long libver;
+#endif
 
     PySSL_BEGIN_ALLOW_THREADS
     if (proto_version == PY_SSL_VERSION_TLS1)