pgsql: Add const qualifier to tzn returned by timestamp2tm()

Add const qualifier to tzn returned by timestamp2tm()

The tzn value might come from tm->tm_zone, which libc declares as
const, so it's prudent that the upper layers know about this as well.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/eb990a2b9efff853d0c68a9cf902b1df23498540

Modified Files
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src/backend/utils/adt/datetime.c | 2 +-
src/backend/utils/adt/formatting.c | 2 +-
src/backend/utils/adt/timestamp.c | 8 ++++----
src/backend/utils/adt/xml.c | 2 +-
src/include/utils/timestamp.h | 2 +-
src/interfaces/ecpg/pgtypeslib/timestamp.c | 4 ++--
6 files changed, 10 insertions(+), 10 deletions(-)