Enable extra-traits feature on syn dependency

As it turns out a Debug implementation for the syn::NestedMeta type is
only available if the 'extra-traits' feature of the crate is enabled.
That is implicitly the case for edition 2018 projects, where resolver v1
is used, which unifies features between regular and dev-dependencies.
However, if the crate is used from an edition 2021 project, resolver v2
is active and it will compile syn with only the explicitly specified
features -- which do not include 'extra-traits'. This may cause a build
failure.
This change fixes the problem by explicitly enabling the 'extra-traits'
feature. Unfortunately, we do not yet want to switch to using edition
2021 or resolver v2 for this crate ourselves -- which would allow us to
detect problems like this in CI moving forward -- as that involves yet
another (arguably significant) minimum supported Rust version bump. So
for now this fix is all we have.

CHANGELOG.md

@@ -1,3 +1,8 @@
+Unreleased
+----------
+- Fixed potential build failure when used from edition 2021 crates
+
+
 0.2.9
 -----
 - Added support for inner `#[test]` attribute arguments

Cargo.toml

@@ -37,7 +37,7 @@ log = []
 [dependencies]
 proc-macro2 = "1.0"
 quote = {version = "1.0"}
-syn = {version = "1.0", features = ["full"]}
+syn = {version = "1.0", features = ["extra-traits", "full"]}
 
 [dev-dependencies]
 env_logger = {version = "0.9", default-features = false}