pgsql: const-qualify ItemPointer comparison functions

const-qualify ItemPointer comparison functions

Add const qualifiers to ItemPointerEquals() and ItemPointerCompare().
This will allow further changes up the stack.  It also complements
commit aeb767ca0b0, as we now have all of itemptr.h appropriately
const-qualified.

Author: Chao Li <li.evan.chao@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAEoWx2nBaypg16Z5ciHuKw66pk850RFWw9ACS2DqqJ_AkKeRsw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3479a0f823d5a6c3ec61dff6b11ea5250389e56a

Modified Files
--------------
src/backend/storage/page/itemptr.c | 4 ++--
src/include/storage/itemptr.h      | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)