Add OPENSSL_cleanup to tls_shutdown function

This prevents a direct leak in OPENSSL_init_crypto (called from
OPENSSL_init_ssl).

Add shim version of OPENSSL_cleanup because it is missing in LibreSSL on
OpenBSD.

lib/isc/openssl_shim.c

@@ -160,3 +160,10 @@ OPENSSL_init_ssl(uint64_t opts, const void *settings) {
 	return (1);
 }
 #endif
+
+#if !HAVE_OPENSSL_CLEANUP
+void
+OPENSSL_cleanup(void) {
+	return;
+}
+#endif