pgsql: Fix function-defined-but-not-used warning.

Fix function-defined-but-not-used warning.

Buildfarm member jacana (MinGW) has been complaining that
get_iso_localename is defined but not used.  This is evidently
fallout from the recent removal of VS2013 support in pg_locale.c.
Rearrange the #ifs so that get_iso_localename and its subroutine
search_locale_enum won't get built on MinGW.

I also noticed that a comment in get_iso_localename cross-
referenced a comment in IsoLocaleName that isn't there anymore.
Put back what I think is the referenced material.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5c7121bcf8dbe94344742a5cd0fa0f016b7a76a1

Modified Files
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src/backend/utils/adt/pg_locale.c | 24 ++++++++++++++++++------
1 file changed, 18 insertions(+), 6 deletions(-)