x86: don't include xen/xen.h in <asm/io.h> unless XEN is enabled

Dmitry Kasatkin reports:
  "kernel-devel package with kernel headers have no <include/xen>
   directory if XEN is disabled.  Modules which inclide asm/io.h won't
   compile.

   XEN related content is behind the CONFIG_XEN flag in the io.h.  And
   <xen/xen.h> should be also behind CONFIG_XEN flag."

So move the include of <xen/xen.h> down into the section that is
conditional on CONFIG_XEN.
Reported-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/io.h

@@ -40,8 +40,6 @@
 #include <linux/compiler.h>
 #include <asm/page.h>
 
-#include <xen/xen.h>
-
 #define build_mmio_read(name, size, type, reg, barrier) \
 static inline type name(const volatile void __iomem *addr) \
 { type ret; asm volatile("mov" size " %1,%0":reg (ret) \
@@ -334,6 +332,7 @@ extern void fixup_early_ioremap(void);
 extern bool is_early_ioremap_ptep(pte_t *ptep);
 
 #ifdef CONFIG_XEN
+#include <xen/xen.h>
 struct bio_vec;
 
 extern bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,