[Backtracing] Fix Windows build.

While I was doing this, it turns out Saleem was fixing things to avoid
having to patch the Windows include directories, which is awesome but
necessitates an extra change to the backtracing stuff to make the build
not fail on Windows.

rdar://105409147

stdlib/public/libexec/swift-backtrace/Utils.swift

@@ -51,6 +51,8 @@ internal func parseUInt64<S: StringProtocol>(_ s: S) -> UInt64? {
   }
 }
 
+#if os(macOS)
+
 struct PosixError: Error {
   var errno: Int32
 
@@ -137,3 +139,4 @@ internal func spawn(_ path: String, args: [String]) throws {
   }
 }
 
+#endif // os(macOS)