x86: remove duplicated extern force_iommu

Both iommu.h and dma-mapping.h have extern force_iommu. The latter
doesn't need to do.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

include/asm-x86/dma-mapping.h

@@ -15,7 +15,6 @@ extern dma_addr_t bad_dma_address;
 extern int iommu_merge;
 extern struct device x86_dma_fallback_dev;
 extern int panic_on_overflow;
-extern int force_iommu;
 
 struct dma_mapping_ops {
 	int             (*mapping_error)(struct device *dev,