uxser: add a missing uxsel_del.

If we close a serial port fd, we shouldn't leave it active in uxsel.
That never ends well.

(cherry picked from commit 6b77fc627a)

unix/uxser.c

@@ -333,6 +333,7 @@ static const char *serial_init(Seat *seat, Backend **backend_handle,
 static void serial_close(Serial *serial)
 {
     if (serial->fd >= 0) {
+        uxsel_del(serial->fd);
         close(serial->fd);
         serial->fd = -1;
     }