pgsql: Cause TestLib.pm to define $windows_os in all branches.

Cause TestLib.pm to define $windows_os in all branches.

Back-port of a part of commit 690ed2b76ab91eb79ea04ee2bfbdc8a2693f2a37 that
I'd depended on without realizing that it was only added recently.  Since
it seems entirely likely that other such tests will need to be back-patched
in future, providing the flag seems like a better answer than just putting
a test in-line.

Per buildfarm.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/06dd4b44fbcef0297acc0fbb1efe311900310272

Modified Files
--------------
src/test/perl/TestLib.pm |    3 +++
1 file changed, 3 insertions(+)