Fix compilation error due to syntex mismatched types

[emef]

Error:

build.rs:27:37: 27:50 error: mismatched types:
expected `&mut syntex::Registry`,
found `&mut inner::syntex::Registry`
(expected struct `syntex::Registry`,
found struct `inner::syntex::Registry`) [E0308]
build.rs:27             serde_codegen::register(&mut registry);
                                            ^~~~~~~~~~~~~

Solution is to use codegen without explicit dependency on syntex, see: https://users.rust-lang.org/t/here-is-how-to-avoid-being-broken-by-syntex-updates/6189?u=dtolnay

[jimmycuadra]

I think only recent versions of serde_codegen support this, so we'll need to bump the required version to the latest one, too.

[jimmycuadra]

Thank you!