pgsql: Mark a static array "const" to move a few bytes from the "data"

Log Message:
-----------
Mark a static array "const" to move a few bytes from the "data" segment
to the "text" segment. It would be possible to mark the elements of the
array "const" as well, but this would require multiple API changes and
does not seem to be worth the notational inconvenience.

Modified Files:
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pgsql/src/backend/catalog:
heap.c (r1.287 -> r1.288)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.287&r2=1.288)