add missing C# windows presubmit.cfg file (#5894)

kokoro/windows/csharp/presubmit.cfg

@@ -0,0 +1,5 @@
+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/windows/csharp/build.bat"
+timeout_mins: 1440