ntlm: mark deliberate switch case fall-through

Coverity CID 1317367, "Missing break in switch"

lib/curl_ntlm.c

@@ -216,7 +216,7 @@ CURLcode Curl_output_ntlm(struct connectdata *conn, bool proxy)
     /* connection is already authenticated,
      * don't send a header in future requests */
     ntlm->state = NTLMSTATE_LAST;
-
+    /* fall-through */
   case NTLMSTATE_LAST:
     Curl_safefree(*allocuserpwd);
     authp->done = TRUE;