x86/PCI: build amd_bus.o only when CONFIG_AMD_NB=y

We only need amd_bus.o for AMD systems with PCI.  arch/x86/pci/Makefile
already depends on CONFIG_PCI=y, so this patch just adds the dependency
on CONFIG_AMD_NB.

Cc: Yinghai Lu <yinghai@kernel.org>
Cc: stable@kernel.org	# 2.6.34+ (needs adjustment for k8 -> amd rename)
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>