Kreeblah reports that clang now warns for powerup.cpp due to a missing
__attribute_format_printf. This was incorrectly dropped in
ffb653c0b8. gcc did not warn, but clang
now warns. Add back the attribute.
Fixes: ffb653c0b8 ("Pass control_info & to various functions")
Reported-by: Kreeblah <https://github.com/dxx-rebirth/dxx-rebirth/pull/547>