Add syn to workspace hack.

Cargo's syn now has the `full` feature, so add it here to keep it in sync.

Author: ehuss

src/tools/rustc-workspace-hack/Cargo.toml

@@ -69,8 +69,9 @@ rand = { version = "0.6.1", features = ["i128_support"] }
 serde = { version = "1.0.82", features = ['derive'] }
 serde_json = { version = "1.0.31", features = ["raw_value"] }
 smallvec = { version = "0.6", features = ['union', 'may_dangle'] }
-scopeguard = { version = "0.3.3", features = ["use_std", "default"]}
-byteorder = { version = "1.2.7", features = ["i128"]}
+scopeguard = { version = "0.3.3", features = ["use_std", "default"] }
+byteorder = { version = "1.2.7", features = ["i128"] }
+syn = { version = "0.15.35", features = ["extra-traits", "full"] }
 
 
 [target.'cfg(not(windows))'.dependencies]