unison package missed the "unison-fsmonitor" in Cygwin

[Andrew Schulman]

> Hi,
> The unison-fsmonitor is missed in Cygwin, according to its official page, the fsmonitor should support Cygwin.
> https://github.com/bcpierce00/unison/blob/master/src/fsmonitor/watchercommon.ml
> 
> Please consider ship it with unison package in Cygwin. Thanks.

Thanks for pointing me to that. I wasn't aware of it. I'll take a look and
include it in a new release if it works for Cygwin. Andrew