libsysdecode: Include required headers in sysdecode.h
Make sysdecode.h self-contained rather than forcing all consumers to
include dependencies. No functional change intended.
Reviewed by: pauamma_gundo.com, jhb, emaste
Sponsored by: The FreeBSD Foundation
(cherry picked from commit 354efc4c94078c3c83ea45d467c6377f3f92926e)