staging: comedi: ni_670x: fix compile on ARCH=arm

staging: comedi: ni_670x: fix compile on ARCH=arm

The kbuild test robot reported a build error on ARCH=arm about
implicit declaration of the functions 'kmalloc' and 'kfree'.

On ARCH=x86 the slab.h header is indirectly included by pci.h,
this does not occur on ARCH=arm.

Include slab.h directly instead of relying on some other header
to include it.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: kbuild test robot <fengguang.wu@intel.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>