libsysdecode: Use consistent include path

mkioctls should look at the same set of headers as mktables does.

MFC after: 1 week
Fixes: 139d114acc7b ("libsysdecode use MKTABLES_INCLUDEDIR")
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D54106