sound: Update some LICENSE headers

The following commits introduced substantial changes to pcm/dsp.c,
pcm/sndstat.c and pcm/sound.c.

9da3b645dbaaad724d524727d003fed7be05ff7c ("sound: Move
sndstat_prepare_pcm() to pcm/sndstat.c and remove
sndstat_entry->handler")
e8c0d15a64fadb4a330f2da7244becaac161bb70 ("sound: Get rid of snd_clone
and use DEVFS_CDEVPRIV(9)")

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44757

(cherry picked from commit 4f854658c5d6ca98c822491991052574d948617d)