x86/platform: Add kvmconfig to the phony targets

... so as not to disable it with a file of the same name in the
toplevel build directory.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1371801891-23618-1-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>