libsysdecode: use local sysdecode.h

Replace <sysdecode.h> with "sysdecode.h" so local builds use the in-tree header
in lib/libsysdecode instead of a stale installed copy in /usr/include, avoiding
build failures after updating sysdecode.h.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored-by: Google LLC (GSoC 2026)
Reviewed by: kp
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2338