x86: pci-dma.c declare iommu_bio_merge before they get used

moved iommu_bio_merge from io_64.h to io.h because it is required for both.
Signed-off-by: NJaswinder Singh <jaswinder@infradead.org>

include/asm-x86/io.h

@@ -73,6 +73,8 @@ build_mmio_write(__writeq, "q", unsigned long, "r", )
 #define writeq writeq
 #endif
 
+extern int iommu_bio_merge;
+
 #ifdef CONFIG_X86_32
 # include "io_32.h"
 #else

include/asm-x86/io_64.h

@@ -233,7 +233,6 @@ void memset_io(volatile void __iomem *a, int b, size_t c);
 
 #define flush_write_buffers()
 
-extern int iommu_bio_merge;
 #define BIO_VMERGE_BOUNDARY iommu_bio_merge
 
 /*