tools headers: Add stub definition for __iomem

Add an empty definition for __iomem so that kernel headers that use
__iomem can be imported into tools/include/ with less modifications.

Acked-by: Shuah Khan <[e-mail]>
Signed-off-by: David Matlack <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alex Williamson <[e-mail]>

tools/include/linux/compiler.h

···
 # define __force
 #endif
 
+#ifndef __iomem
+# define __iomem
+#endif
+
 #ifndef __weak
 # define __weak			__attribute__((weak))
 #endif