ANDROID: staging: 'vsoc' driver build fix.

This driver uses kmalloc/kcalloc/kfree without including <linux/slab.h>.
Affects some defconfigs.

Fixes: 692acd7b ("FROMLIST: staging: Android: Add 'vsoc' driver for
                     cuttlefish.")
Bug: 70214720
Change-Id: Ic03348415e0ba6c36387ed09551de038063f129c
Signed-off-by: Alistair Strachan <astrachan@google.com>