Update to swift-syntax 509.0.2 (#56)

Increases the minimum swift-syntax version to 509.0.2 in order to pick
up a bug fix needed for linux testing, see: swiftlang/swift-syntax#2321 for
more details.

Author: rauhul

Package.resolved

@@ -17,7 +17,7 @@ var package = Package(
     .library(name: "MMIO", targets: ["MMIO"])
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax.git", from: "509.0.1")
+    .package(url: "https://github.com/apple/swift-syntax.git", from: "509.0.2")
   ],
   targets: [
     .target(