remove serde_derive dependency

[ignatenkobrain]

It was pinned to be used with standalone-syn. Since we have switched
back to syn, it is no longer needed.

Signed-off-by: Igor Gnatenko i.gnatenko.brain@gmail.com

[cuviper]

It was pinned so that the version of syn used by serde-derive would be older, distinct from the version used by cbindgen itself. This is because serde-derive with current syn will enable the proc-macro feature (by default), which dynamically links it to compiler libraries. That's fine for transient derive plugins, but then cbindgen with that same syn is stuck also linked to compiler libraries. Cargo doesn't have a way to say you want the same syn built different ways, so you could also have it without proc-macro.

For rustup users, this is a problem because your installed ~/.cargo/bin/cbindgen will depend on the exact compiler version activated, and will fail when that one is not in LD_LIBRARY_PATH. Similarly for your Fedora build, you've got a few unsatisfied library Requires, as the rust rpm explicitly filters its libraries from auto-Provides.

[eqrion]

I'm going to close this as there's no good path forward. Once #203 is resolved we can remove the pin.