allow(deprecated) for glib::CallbackGuard, until we formally remove it

Rust 1.24 made it so that a panic!() won't unwind across FFI
boundaries: we won't unwind if we panic inside a Rust function
declared extern "C".

rust-lang/rust#46833

However, gnome-class does not formally mandate a particular Rust
version; we've been running on the assumption that we are running on
nightly, or "recent enough".  For now, just supress the deprecation
warnings from glib-rs, until we formalize our requirements for the
rustc version.

src/glib_utils.rs

@@ -41,7 +41,11 @@ pub fn tokens_ParentClassFfi(class: &ast::Class) -> Tokens {
 }
 
 pub fn glib_callback_guard() -> Tokens {
+    // FIXME: remove this function if we formally declare that
+    // gnome-class will require Rust 1.24 or later?  That version made
+    // glib::CallbackGuard obsolete.
     quote_cs! {
+        #[allow(deprecated)]
         let _guard = glib::CallbackGuard::new();
     }
 }