pgsql: Skip SectionMemoryManager.h in cpluspluscheck.

Skip SectionMemoryManager.h in cpluspluscheck.

Commit 9044fc1d45a0 skipped SectionMemoryManager.h in headerscheck, and
by extension also cpluspluscheck, because it's C++ and would fail both
tests.  That worked in master and REL_17_STABLE due to 7b8e2ae2fd3b, but
older branches have a separate cpluspluscheck script.  We need to copy
the filtering rule into there too.

This problem was being reported by CI's CompilerWarnings task in the 15
and 16 branches, but was a victim of alert fatigue syndrome (unrelated
problems in the back-branches).

Fix 16, and back-patch to 13, as those are the live branches that have a
separate cpluspluscheck script.

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b792aa989bb80ae924867e311d456c9875411828

Modified Files
--------------
src/tools/pginclude/cpluspluscheck | 3 +++
1 file changed, 3 insertions(+)