x86/gart: Unexport gart_iommu_aperture

I wrongly exported gart_iommu_aperture in the commit
42590a75. It's not necessary so
let's unexport it.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Joerg Roedel <joerg.roedel@amd.com>
LKML-Reference: <20100215113241P.fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>