resolve: Silence build warning, when systemd is built without libidn

HAVE_IDN is not defined when systemd is build without it

src/resolve/test-dnssec-complex.c

@@ -220,7 +220,7 @@ int main(int argc, char* argv[]) {
         test_hostname_lookup(bus, "poettering.de", AF_INET, NULL);
         test_hostname_lookup(bus, "poettering.de", AF_INET6, NULL);
 
-#if HAVE_LIBIDN
+#ifdef HAVE_LIBIDN
         /* Unsigned A with IDNA conversion necessary */
         test_hostname_lookup(bus, "pöttering.de", AF_UNSPEC, NULL);
         test_hostname_lookup(bus, "pöttering.de", AF_INET, NULL);