cpp: include config.h when necessary

tools/dhtnode.cpp

@@ -18,6 +18,9 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program. If not, see <https://www.gnu.org/licenses/>.
  */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
 
 #include "tools_common.h"
 extern "C" {