libkernel: CpuOps: add 'static trait bound

The CpuOps trait should never hold a reference to any other object. In
fact, it should be a singleton object which is constructed at kernel
boot and live for the entire lifetime of the system. Therefore, it's
safe to add the `'static` trait bound.

libkernel/src/lib.rs

@@ -21,7 +21,7 @@ pub mod sync;
 
 extern crate alloc;
 
-pub trait CpuOps {
+pub trait CpuOps: 'static {
     /// Returns the ID of the currently executing core.
     fn id() -> usize;