kcmp: Move kcmp.h into uapi

kcmp.h appears to be part of the API, it's documented in kcmp(2), and
the selftests/kcmp code uses it. So move it to uapi so it's actually
exported.

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>