x86_64: pci-gart_64.c iommu_fullflush should be static

Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:

  arch/x86/kernel/pci-gart_64.c:55:5: warning: symbol 'iommu_fullflush' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>