Implement TmfRangeSynchSignal toString

Implement TmfRangeSynchSignal toString

I found it sad that TmfTimeSynchSignal had its toString but not
TmfRangeSynchSignal. This commit fixes the injustice.

Example of the result:

TmfRangeSynchSignal [source=\
org.eclipse.tracecompass.tmf.ui.views.histogram.HistogramView@3a7b058e, \
range=TmfTimeRange [fStartTime=03:02:54.940 166 462, \
fEndTime=03:02:54.977 325 374]]

Change-Id: Ic3accae7610e454736497251c170fbdecb9b83fb
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Reviewed-on: https://git.eclipse.org/r/38067
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>