i2c-i801: Include <linux/slab.h>

Commit 5a0e3ad6af8660be21ca98a971cd00f331318c05 added direct inclusion
of <linux/slab.h> to those source files that appeared to need it, but
somehow missed this.  On most architectures <linux/slab.h> is still
indirectly included, but there are exceptions such as alpha.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jean Delvare <khali@linux-fr.org>