meson: Fix meson warning

meson: Fix meson warning

    WARNING: You should add the boolean check kwarg to the run_command call.
             It currently defaults to false,
             but it will default to true in meson 2.0.

Introduced by commit bc46104fc9a.

(This only happens in the msvc branch.  All the other run_command
calls are ok.)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: http://www.postgresql.org/message-id/flat/42e13eb0-862a-441e-8d84-4f0fd5f6def0%40eisentraut.org