pgsql: Add missing parenthesis to max item size macro.

Add missing parenthesis to max item size macro.

Oversight in commit 92f37505, per buildfarm.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b2fe783aecadb42e17868787bb45aa6328796d2a

Modified Files
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src/backend/access/nbtree/nbtdedup.c | 3 +++
src/include/access/nbtree.h | 8 ++++----
2 files changed, 7 insertions(+), 4 deletions(-)