Add PGDLLEXPORTS to some plpgsql function declarations
After -fvisibility=hidden was added by
089480c07705, plpgsql_check no
longer works; this quick hack fixes it. It would be better to
restructure the plpgsql.h header so that this doesn't look as random,
but we can leave that for another day.
Reported-by: Pavel Stehule
Discussion: https://postgr.es/m/CAFj8pRAFxc3-SHMD3URU09JZXEKY3W-RwXKp8xPEnEq8rrka7w@mail.gmail.com