kmsro: Silence warning if missing

kmsro: Silence warning if missing

Regardless of whether the build uses kmsro, kmsro is the default driver
descriptor when the static loader is used. Thus, in an edge case where
the static loader is used, no static targets are loaded, and kmsro is
not compiled, a spurious warning is printed. There's no harm in
executing the stub function in this case, but it's not "an error" to not
have kmsro in the build; the driver missing warning should not printed
kmsro.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>