powerpc: Wireup the kcmp syscall to sys_ni

Since kmp takes 2 unsigned long args there should be a compat wrapper.
Since one isn't provided I think it's safer just to hook this up to not
implemented.  If we need it later we can do it properly then.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>