pgsql: Add some const qualifications

Add some const qualifications

Add some const qualifications afforded by the previous change that
added a const qualification to PageAddItemExtended().

Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://www.postgresql.org/message-id/flat/c75cccf5-5709-407b-a36a-2ae6570be766(at)eisentraut(dot)org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/10b5bb3bffaee8d136d970c5b2ec1f21592b581b

Modified Files
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src/backend/access/brin/brin_pageops.c | 2 +-
src/backend/access/gin/ginfast.c | 2 +-
src/backend/access/nbtree/nbtinsert.c | 4 ++--
src/backend/access/nbtree/nbtsort.c | 4 ++--
src/include/access/brin_pageops.h | 2 +-
5 files changed, 7 insertions(+), 7 deletions(-)