Add missing feature to syn (#5547)
### Description
Cargo check currently fails with:
```
error[E0277]: `syn::Type` doesn't implement `Debug`
--> crates/turbo-tasks-macros-shared/src/primitive_input.rs:8:5
|
6 | #[derive(Debug)]
| ----- in this derive macro expansion
7 | pub struct PrimitiveInput {
8 | pub ty: Type,
| ^^^^^^^^^^^^ `syn::Type` cannot be formatted using `{:?}` because it doesn't implement `Debug`
|
= help: the trait `Debug` is not implemented for `syn::Type`
= note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0277`.
error: could not compile `turbo-tasks-macros-shared` (lib) due to previous error
```
This is because because turbo-tasks-macros-shared (which is missing a
syn feature to enable the debug trait) is part of the workspace default
members, while turbo-tasks-macros (which has the feature) isn’t.
### Testing Instructions
`cargo check`
