Do not take &mut in Tracer::trace_static

Summary:
When implementing `Trace` for `HashMap`, `iter_mut` returns `&K`, which cannot be fed to `trace_static`.

Like in the following diff D53944118.

Reviewed By: JakobDegen

Differential Revision: D53944197

fbshipit-source-id: afca74dbb995737970ba17dd2894e1dd2c295666

starlark/src/values/layout/heap/heap_type.rs

@@ -893,7 +893,7 @@ impl<'v> Tracer<'v> {
     /// Helper function to annotate that this field has been considered for tracing,
     /// but is not relevant because it has a static lifetime containing no relevant values.
     /// Does nothing.
-    pub fn trace_static<T: 'static>(&self, value: &mut T) {
+    pub fn trace_static<T: 'static>(&self, value: &T) {
         // Nothing to do because T can't contain the lifetime 'v
         let _ = value;
     }