powerpc: Fix distclean with Makefile.postlink

Makefile.postlink always includes include/config/auto.conf, however
this file is not present in a clean kernel tree, causing make to fail:

  $ git clone linuxppc.git
  $ cd linuxppc.git
  $ make distclean
  arch/powerpc/Makefile.postlink:10: include/config/auto.conf: No such file or directory
  make[1]: *** No rule to make target `include/config/auto.conf'.  Stop.
  make: *** [vmlinuxclean] Error 2

Equally running 'make distclean; make distclean' will trip the error case.

Change the inclusion such that file not being found does not trigger an error.

Fixes: f188d052 ("powerpc: Use the new post-link pass to check relocations")
Reported-by: Mircea Pop <mircea.pop@nxp.com>
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>