LinuxKPI: add _devcd_free_sgtable() to devcoredump.h

It may be that once we implement freeing of chained tables
_devcd_free_sgtable() will become our _lkpi_dev_coredumpsg_free()
but further investigations need to happen. For now make an
updated iwlwifi driver happy which should not need more.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D50006