io-mapping: move asm include inside the config option

nouveau starting using these APIs, the first on non-x86 hw, and this
include isn't required on anything with real amounts of vmalloc space.

this fixes a build problem on powerpc.
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/linux/io-mapping.h

@@ -22,7 +22,6 @@
 #include <linux/slab.h>
 #include <asm/io.h>
 #include <asm/page.h>
-#include <asm/iomap.h>
 
 /*
  * The io_mapping mechanism provides an abstraction for mapping
@@ -33,6 +32,8 @@
 
 #ifdef CONFIG_HAVE_ATOMIC_IOMAP
 
+#include <asm/iomap.h>
+
 struct io_mapping {
 	resource_size_t base;
 	unsigned long size;