x86: Add iommu_init to x86_init_ops, fix build

Most of the time x86_init.h is included in pci-dma.c - but not always,
leading to this rare build failure:

arch/x86/kernel/pci-dma.c:296: error: 'x86_init' undeclared (first use in this function)

So include asm/x86_init.h explicitly.

Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: chrisw@sous-sol.org
Cc: dwmw2@infradead.org
Cc: joerg.roedel@amd.com
Cc: muli@il.ibm.com
LKML-Reference: <1257849980-22640-2-git-send-email-fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/pci-dma.c

@@ -11,6 +11,7 @@
 #include <asm/gart.h>
 #include <asm/calgary.h>
 #include <asm/amd_iommu.h>
+#include <asm/x86_init.h>
 
 static int forbid_dac __read_mostly;