pgsql: Use correct PATH separator for Cygwin in pg_regress.c.

Use correct PATH separator for Cygwin in pg_regress.c.

This has been broken for years, and I'm not sure why it has not been
noticed before, but now a very modern Cygwin breaks on it, and the fix
is clearly correct. Backpatching to all live branches.

Branch
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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/158e04274776a4648f6ad99376d98f0fc893500c

Modified Files
--------------
src/test/regress/pg_regress.c |    4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)