[fidl][rust] Put #[allow(dead_code)] on empty services

Since rust-lang/rust#118297, Rust treats unused
tuple struct fields as dead code. This CL adds an #[allow(dead_code)] to
silence the warning.

Bug: 318827209
Change-Id: I6858165a085a0ef508722c1ce38f3d95866d2867
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/972128
Reviewed-by: David Koloski <dkoloski@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Mitchell Kember <mkember@google.com>

Author: Mitchell Kember

tools/fidl/fidlgen_rust/codegen/service.tmpl

@@ -58,7 +58,7 @@ impl fidl::endpoints::ServiceRequest for {{ .Name }}Request {
 ///{{ . }}
 {{- end }}
 #[cfg(target_os = "fuchsia")]
-pub struct {{ .Name }}Proxy(Box<dyn fidl::endpoints::MemberOpener>);
+pub struct {{ .Name }}Proxy({{ if not .Members }}#[allow(dead_code)] {{ end }}Box<dyn fidl::endpoints::MemberOpener>);
 
 #[cfg(target_os = "fuchsia")]
 impl fidl::endpoints::ServiceProxy for {{ .Name }}Proxy {

tools/fidl/fidlgen_rust/goldens/service.rs.golden

@@ -656,7 +656,7 @@ impl fidl::endpoints::ServiceRequest for EmptyServiceRequest {
     }
 }
 #[cfg(target_os = "fuchsia")]
-pub struct EmptyServiceProxy(Box<dyn fidl::endpoints::MemberOpener>);
+pub struct EmptyServiceProxy(#[allow(dead_code)] Box<dyn fidl::endpoints::MemberOpener>);
 
 #[cfg(target_os = "fuchsia")]
 impl fidl::endpoints::ServiceProxy for EmptyServiceProxy {