Adjust rustc-workspace-hack

Cargo.lock

@@ -3393,6 +3393,7 @@ dependencies = [
  "serde",
  "serde_json",
  "smallvec",
+ "syn 0.15.35",
  "url 2.1.0",
  "winapi 0.3.6",
 ]

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

@@ -64,7 +64,7 @@ serde = { version = "1.0.82", features = ['derive'] }
 serde_json = { version = "1.0.31", features = ["raw_value"] }
 smallvec = { version = "0.6", features = ['union', 'may_dangle'] }
 url = { version = "2.0", features = ['serde'] }
-
+syn = { version = "0.15", features = ['full'] }
 
 [target.'cfg(not(windows))'.dependencies]
 openssl = { version = "0.10.12", optional = true }