x86, iommu: replace CONFIG_IOMMU with CONFIG_GART_IOMMU in iommu.h

Hmm, looks like it would be nice to have more cleanups of iommu.h and
gart.h.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/asm-x86/iommu.h b/include/asm-x86/iommu.h
index 07862fd..cb7e989 100644
--- a/include/asm-x86/iommu.h
+++ b/include/asm-x86/iommu.h
@@ -5,7 +5,7 @@
 extern void no_iommu_init(void);
 extern int force_iommu, no_iommu;
 extern int iommu_detected;
-#ifdef CONFIG_IOMMU
+#ifdef CONFIG_GART_IOMMU
 extern void gart_iommu_init(void);
 extern void gart_iommu_shutdown(void);
 extern void __init gart_parse_options(char *);