cbus: fix comilation breakage

Just tried to build n8x0_defconfig on the current master and
got cbus failing to compile due to implicit kzalloc() (and
others from slab.h) references. I'm not sure that including
slab.h directly is the right thing to do here, though.

Signed-off-by: Alexander Shishkin <ash@koowaldah.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>