io/channel-watch: Drop the unnecessary cast

There is no need to do a type cast on ssource->socket as it is
already declared as a SOCKET.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>