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
------
REL8_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5773fac1606cdeb3fa60f89eb8c20ef5e904663a

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