pg_upgrade copy_file not needed on Win32

I have applied the attached patch which prevents copy_file() from being
compiled on Win32, per report from Andrew Dunstan. copy_file() is not
used on Win32 and generates a warning.

--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +