rust: Make the io_mem crate depend on CONFIG_HAS_IOMEM

The kernel iomem functions are not available on all architectures, in
all configurations, so hide the rust bindings behind CONFIG_HAS_IOMEM.

This is required for rust to work under UML.

Signed-off-by: David Gow <davidgow@google.com>

Author: sulix

rust/kernel/lib.rs

@@ -87,6 +87,7 @@ pub mod sync;
 pub mod sysctl;
 
 pub mod io_buffer;
+#[cfg(CONFIG_HAS_IOMEM)]
 pub mod io_mem;
 pub mod iov_iter;
 pub mod of;