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: Jaswinder Singh <jaswinder@infradead.org>
diff --git a/include/asm-x86/io.h b/include/asm-x86/io.h
index bf5d629..a145210 100644
--- a/include/asm-x86/io.h
+++ b/include/asm-x86/io.h
@@ -73,6 +73,8 @@
 #define writeq writeq
 #endif
 
+extern int iommu_bio_merge;
+
 #ifdef CONFIG_X86_32
 # include "io_32.h"
 #else