tmf: Fix calculation of available alignment width in TimeGraphCombo

tmf: Fix calculation of available alignment width in TimeGraphCombo

Before the fix the vertical scroll bar was not considered when
calculating the avalible alignment width.

To observe the impact of the incorrect aligment value open just
HistogramView and ControlFlowView for a kernel trace without the fix.
Then the right margin of the Histogram view is not correct.

Change-Id: Ia54e2c3f03fcc22def4706b778518353515c7897
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/47852
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>