rust: add unstable feature associated_type_defaults

Previously imported through "*RFL import: kernel::platform".

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

rust/kernel/lib.rs

@@ -13,6 +13,7 @@
 
 #![no_std]
 #![feature(arbitrary_self_types)]
+#![feature(associated_type_defaults)]
 #![feature(coerce_unsized)]
 #![feature(const_refs_to_cell)]
 #![feature(dispatch_from_dyn)]