[media] radio-rtrack2: add missing slab.h

Missed this one, this patch should solve this issue:

http://www.spinics.net/lists/linux-media/msg45880.html



Slab.h was added to all the other isa drivers, but not radio-rtrack2.c.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>