style: Update cbindgen.

This cleans up the pattern of "Use a private dtor so that the helper functions
do the right thing" by enabling it everywhere using:

  https://github.com/eqrion/cbindgen/pull/377

It also caught some uninitialized value issues.

I think they're mostly harmless since we zero-initialize our structs:

https://searchfox.org/mozilla-central/rev/325c1a707819602feff736f129cb36055ba6d94f/servo/components/style/properties/gecko.mako.rs#632

And since we override the clip rect, which is the other bit of code that was
failing to build with this change.

Differential Revision: https://phabricator.services.mozilla.com/D43491

components/style/values/specified/motion.rs

@@ -15,7 +15,6 @@ use style_traits::{ParseError, StyleParseErrorKind};
 /// The offset-path value.
 ///
 /// https://drafts.fxtf.org/motion-1/#offset-path-property
-/// cbindgen:derive-tagged-enum-copy-constructor=true
 #[derive(
     Animate,
     Clone,