selftests/kcmp: Don't include kernel headers

The kcmp test mucks with the include path to bring in the kernel
headers, and x86 headers too for reasons that are not clear.

Now that kcmp.h is exported none of that should be necessary.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>