x86_64: change _map_single to static in pci_gart.c etc

This function is called via dma_ops->.., so change it to static

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>