silence unused m variable warning in #[hdl_module] with an empty body.

crates/fayalite-proc-macros-impl/src/module.rs

@@ -377,7 +377,7 @@ impl ModuleFn {
             module_kind,
             vis,
             sig,
-            block,
+            mut block,
             struct_generics,
             the_struct,
         } = match self.0 {
@@ -439,6 +439,12 @@ impl ModuleFn {
         body_sig
             .inputs
             .insert(0, parse_quote! { m: &::fayalite::module::ModuleBuilder });
+        block.stmts.insert(
+            0,
+            parse_quote! {
+                let _ = m;
+            },
+        );
         let body_fn = ItemFn {
             attrs: vec![],
             vis: Visibility::Inherited,