pgsql: Exclude common/int128.h from cpluspluscheck

Exclude common/int128.h from cpluspluscheck

It uses static assertions, which are not supported under C++ in this
branch.

This change only goes into the 9.4 branch, because 9.5 and beyond will
primarily use the USE_NATIVE_INT128 branch, so cpluspluscheck isn't
bothered.  In PG11 we will have C++ support for static assertions, so
the issue will go away altogether.

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9f050c0b41685e61c7373d7af9f97740b61a316d

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