pgsql: Further fix extended alignment for older g++.

Further fix extended alignment for older g++.

Commit 6ceef9408 was still one brick shy of a load, because it caused
any usage at all of PGIOAlignedBlock or PGAlignedXLogBlock to fail
under older g++. Notably, this broke "headerscheck --cplusplus".
We can permit references to these structs as abstract structs though;
only actual declaration of such a variable needs to be forbidden.

Discussion: https://www.postgresql.org/message-id/3119480.1769189606@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bd9dfac8b121e67d7dd4a9bfecb1474fe6811927

Modified Files
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src/include/c.h | 6 ++++++
1 file changed, 6 insertions(+)