selftests/powerpc: Fix out-of-tree build errors

[Michael Ellerman]

On Fri, 2018-10-19 at 04:06:16 UTC, Michael Ellerman wrote:
> Some of our Makefiles don't do the right thing when building the
> selftests with O=, fix them up.
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/d8a2fe29d3c97038c8efcc328d5e79

cheers