Attempt to fix win32 build of pg_verify_checksums

Magnus Hagander <magnus@hagander.net>

Commit: 3b0b4f31f73a5f45f8e122d826211c13cd2412f7
Author: Magnus Hagander <magnus@hagander.net>
Date: 2018-04-05T20:38:03Z
Releases: 11.0
Attempt to fix win32 build of pg_verify_checksums

S_ISLNK doesn't exist on Win32, instead we should use
pgwin32_is_junction().

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