tmf: Expose the indexTrace() method in the interface

tmf: Expose the indexTrace() method in the interface

Some trace types could decide to do their indexing manually
and such, so expose the already-existing indexTrace() method
up to the ITmfTrace interface.

Change-Id: I1f9724f3cb20b8bbf19b8a5120cc16155b7de173
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/12078
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>