rv/rvgen: fix _fill_states() return type annotation

The _fill_states() method returns a list of strings, but the type
annotation incorrectly specified str. Update the annotation to
list[str] to match the actual return value.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20260223162407.147003-20-wander@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>