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/generics/transform.rs

@@ -151,7 +151,6 @@ fn is_same<N: PartialEq>(x: &N, y: &N) -> bool {
 )]
 #[repr(C, u8)]
 /// A single operation in the list of a `transform` value
-/// cbindgen:derive-tagged-enum-copy-constructor=true
 pub enum GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>
 where
     Angle: Zero,