Fix features in syn dependency

Author: SimonSapin

Cargo.toml

@@ -30,7 +30,7 @@ serde = {version = "1.0", optional = true}
 smallvec = "0.6"
 
 [build-dependencies]
-syn = { version = "0.14", features = ["extra-traits", "fold"] }
+syn = { version = "0.14", features = ["extra-traits", "fold", "full"] }
 quote = "0.6"
 proc-macro2 = "0.4"