setup-environment: Add tracing layer in OPTIONALLAYERS

* We need gstreamer tracing in future. Right now tracing
  layer is only added when we add qt5 layer because tracing
  layer is in its depends. Now we need it even qt5 is not
  added to the build. So added tracing layer in
  setup-environment script so that it automatically added
  to the bbalyers.conf.

Signed-off-by: Noor Ahsan <noor_ahsan@mentor.com>