JitRegister: Fix IsEnabled when using VTune without perf

Set `s_is_enabled` to `true` in `Init` when `USE_VTUNE` is defined so
that `IsEnabled` returns true even if perf isn't being used.

Source/Core/Common/JitRegister.cpp

@@ -34,6 +34,10 @@ static bool s_is_enabled = false;
 
 void Init(const std::string& perf_dir)
 {
+#ifdef USE_VTUNE
+  s_is_enabled = true;
+#endif
+
   if (!perf_dir.empty() || getenv("PERF_BUILDID_DIR"))
   {
     const std::string dir = perf_dir.empty() ? "/tmp" : perf_dir;