pgsql: Move pg_int64 back to postgres_ext.h

Move pg_int64 back to postgres_ext.h

Fix for commit 3c86223c998. That commit moved the typedef of pg_int64
from postgres_ext.h to libpq-fe.h, because the only remaining place
where it might be used is libpq users, and since the type is obsolete,
the intent was to limit its scope.

The problem is that if someone builds an extension against an
older (pre-PG18) server version and a new (PG18) libpq, they might get
two typedefs, depending on include file order. This is not allowed
under C99, so they might get warnings or errors, depending on the
compiler and options. The underlying types might also be
different (e.g., long int vs. long long int), which would also lead to
errors. This scenario is plausible when using the standard Debian
packaging, which provides only the newest libpq but per-major-version
server packages.

The fix is to undo that part of commit 3c86223c998. That way, the
typedef is in the same header file across versions. At least, this is
the safest fix doable before PostgreSQL 18 releases.

Reviewed-by: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/25144219-5142-4589-89f8-4e76948b32db%40eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e56a601e067894002c8bbc19cef5c4ef584ba8ad

Modified Files
--------------
src/include/postgres_ext.h | 5 +++++
src/interfaces/libpq/libpq-fe.h | 3 ---
2 files changed, 5 insertions(+), 3 deletions(-)