pgsql: Add const qualifiers where they are accidentally cast away

Add const qualifiers where they are accidentally cast away

This only produces warnings under -Wcast-qual, but it's more correct
and consistent in any case.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/973e9fb294dc05a384ecae7623923ae53cb81806

Modified Files
--------------
contrib/intarray/_intbig_gist.c      |    2 +-
contrib/ltree/ltree_gist.c           |    4 ++--
src/backend/access/gist/gistproc.c   |   12 ++++++------
src/backend/utils/adt/tsquery_util.c |    2 +-
src/backend/utils/adt/xml.c          |    2 +-
src/backend/utils/misc/guc.c         |    4 ++--
src/bin/psql/mbprint.c               |    8 ++++----
src/include/c.h                      |    2 +-
src/include/utils/pg_crc.h           |    2 +-
9 files changed, 19 insertions(+), 19 deletions(-)