maintainers: add Windows exports script

A script used with meson was missing in the list of files.

Fixes: 4c773788e05a ("build: generate Windows exports file")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>