MINOR: DNS counters: increment valid counter

Valid counter was never incremented.
Now it is.
diff --git a/src/dns.c b/src/dns.c
index 5bc57e5..53e7430 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -219,6 +219,7 @@
 
 		}
 
+		nameserver->counters.valid += 1;
 		resolution->requester_cb(resolution, nameserver, buf, buflen);
 	}
 }