pgsql: Disable some C++ warnings in MSVC

Disable some C++ warnings in MSVC

Flexible array members, as used in many PostgreSQL header files, are
not a C++ feature.  MSVC warns about these.  Disable the
warning.  (GCC and Clang accept them, but they would warn in -pedantic
mode.)

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion:
https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/82a7cbea747c465eee8449587be2d6cc47af82fe

Modified Files
--------------
meson.build | 3 +++
1 file changed, 3 insertions(+)