dns: fix last timestamp handling

Fixes incorrect variable in ticket #2207

In app-layer-dns-tcp.c in the DNSTCPResponseParse function
a variable is set to last_req when it should be last_resp.
This makes it consistent with UDP DNS response parsing.

src/app-layer-dns-tcp.c

@@ -583,7 +583,7 @@ next_record:
     }
 
     if (f != NULL) {
-        dns_state->last_req = f->lastts;
+        dns_state->last_resp = f->lastts;
     }
 
     SCReturnInt(1);