io/channel-watch: Drop a superfluous '#ifdef WIN32'

In the win32 version qio_channel_create_socket_watch() body there is
no need to do a '#ifdef WIN32'.

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>