[AGPGART] Make AGP depend on PCI

Fixes possible compile error in amd64 with pci=n
pointed out by Adrian Bunk.
Signed-off-by: NDave Jones <davej@redhat.com>

drivers/char/agp/Kconfig

 config AGP
 	tristate "/dev/agpgart (AGP Support)" if !GART_IOMMU
 	depends on ALPHA || IA64 || PPC || X86
+	depends on PCI
 	default y if GART_IOMMU
 	---help---
 	  AGP (Accelerated Graphics Port) is a bus system mainly used to