[Skiboot] native build broken for some time

[Dan Horák]

Hi,

when working on the opal-prd.service change I have realized that it's
not possible to build the "external" tools (opal-prd at least) using a
host compiler any more due an conflict between local and system headers.
Bisecting shows
https://github.com/open-power/skiboot/commit/18c9b278e7473204ab5bef69e788a29f92a130c3
as the culprit. I believe the "external" tools like opal-prd should be
buildable using the host compiler as they are expected to run inside
the user OS. I am going to look into it.


	With regards,

		Dan