udpateconninfo: mark variable unused

When compiling without getpeername() or getsockname(), the sockfd
paramter to Curl_udpateconninfo() became unused after commit e91e481612
added ifdef guards.

Closes #3910
Fixes https://curl.haxx.se/dev/log.cgi?id=20190520172441-32196
Reviewed-by: Marcel Raad, Daniel Stenberg

lib/connect.c

@@ -726,6 +726,8 @@ void Curl_updateconninfo(struct connectdata *conn, curl_socket_t sockfd)
     }
 #endif
   }
+#else /* !HAVE_GETSOCKNAME && !HAVE_GETPEERNAME */
+  (void)sockfd; /* unused */
 #endif
 
   /* persist connection info in session handle */