(#8663) Exclude git rev-parse HEAD spec test on Windows

When running spec/integration/parser/compiler_spec.rb, the git command
passed to Puppet::Util.execute is an array containing a single string
["git rev-parse HEAD"]. On Unix, it calls Kernel.exec(*command), but on
Windows it calls Process.create(:command_line =>command), which attempts
to execute the program called "git rev-parse HEAD" and fails:

CreateProcess() failed: The system cannot find the file specified.

This commit marks the test as fails_on_windows, and will be re-enabled
when #8410 is fixed.

Reviewed-by: Nick Lewis <nick@puppetlabs.com>