Cleanup mttest.c : make ssl_method a pointer to const

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/threads/mttest.c

@@ -189,7 +189,7 @@ int main(int argc, char *argv[])
     SSL_CTX *c_ctx = NULL;
     char *scert = TEST_SERVER_CERT;
     char *ccert = TEST_CLIENT_CERT;
-    SSL_METHOD *ssl_method = TLS_method();
+    const SSL_METHOD *ssl_method = TLS_method();
 
     RAND_seed(rnd_seed, sizeof rnd_seed);