x86: amd_iommu.c: prealloc_protection_domains should be static

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

Fixes sparse warning:
arch/x86/kernel/amd_iommu.c:1299:6: warning: symbol 'prealloc_protection_domains' was not declared. Should it be static?
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/amd_iommu.c

@@ -1296,7 +1296,7 @@ static int amd_iommu_dma_supported(struct device *dev, u64 mask)
  * we don't need to preallocate the protection domains anymore.
  * For now we have to.
  */
-void prealloc_protection_domains(void)
+static void prealloc_protection_domains(void)
 {
 	struct pci_dev *dev = NULL;
 	struct dma_ops_domain *dma_dom;