Changes

rcp: Fix 4.20 build after missing c.g.gson version

Fix the recently (yet repeatedly) failing build [1], by adding the now
required com.google.gson to both target files; base this on [2]'s fix.

[1]https://ci.eclipse.org/tracecompass/job/tracecompass-incubator-nightly-2021-06
[2]https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/203585

Change-Id: If873a07bf0877a810c11790b2e17f6c8afcbd470
Signed-off-by: Marco Miller <marco.miller@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass.incubator/org.eclipse.tracecompass.incubator/+/203453
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>