cygwin: Add cocom and patch to build prerequisites in FAQ

cygwin: Add cocom and patch to build prerequisites in FAQ

Add patch, and make cocom unconditional in list of build prerequisites.

Even though the products of these tools are checked in, these tools are
required when building in a fresh git checkout, presumably due to the
order in which git creates the files resulting in timestamps which
indicate that the output of rules using these tools are older than
the inputs.

Addresses: https://cygwin.com/pipermail/cygwin/2021-December/250124.html

Also drop a duplicate sentence about 'fetch sources from git'.