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>

drivers/staging/comedi/drivers/ni_670x.c

@@ -39,6 +39,7 @@ Commands are not supported.
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/interrupt.h>
+#include <linux/slab.h>
 
 #include "../comedidev.h"