Add `syn` to workspace hack.
Add `syn` to workspace hack.

Cargo's syn now has the `full` feature, so add it here to keep it in sync.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust