LinuxKPI: add struct dmi_header and unsupported dmi_walk()

Add a structure definition as well as a dummy dmi_walk for now
which returns an error as not supported. Our current dmi implementation
is special but does not give access to all details but rather only
information from kenv which does not suffice all use cases.

MFC after: 1 week
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D36687