chakrashim: missing return for stubbed method

`TracingController::CurrentTimestampMicroseconds` is declared to return
an `int64_t`, but currently returns nothing.

PR-URL: #468
Reviewed-By: Jimmy Thomson <jithomso@microsoft.com>
Reviewed-By: Hitesh Kanwathirtha <hiteshk@microsoft.com>

deps/chakrashim/src/v8v8.cc

@@ -254,6 +254,7 @@ namespace tracing {
 
   int64_t TracingController::CurrentTimestampMicroseconds() {
     jsrt::Unimplemented("TracingController");
+    return 0;
   }
 
   void TraceConfig::AddIncludedCategory(char const*) {