library: xous: mark alloc as FIXME(static_mut_refs)

The allocator on Xous is now throwing warnings because the allocator
needs to be mutable, and allocators hand out mutable pointers, which
the `static_mut_refs` lint now catches.

Give the same treatment to Xous as wasm, at least until a solution is
devised for fixing the warning on wasm.

Signed-off-by: Sean Cross <sean@xobs.io>

std/src/sys/alloc/xous.rs

@@ -1,3 +1,6 @@
+// FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
+#![allow(static_mut_refs)]
+
 use crate::alloc::{GlobalAlloc, Layout, System};
 
 #[cfg(not(test))]