parisc-agp: fix missing slab.h include

Commit 338e4fab added a missing kfree if the alloc_pci_dev failed
but forgot to include <linux/slab.h> for the definition of
kfree.

Signed-off-by: Kyle McMartin <kyle@redhat.com>

drivers/char/agp/parisc-agp.c

@@ -19,6 +19,7 @@
 #include <linux/klist.h>
 #include <linux/agp_backend.h>
 #include <linux/log2.h>
+#include <linux/slab.h>
 
 #include <asm/parisc-device.h>
 #include <asm/ropes.h>