As discussed in ESC call of 2019-02-21, enable kde5
for the Jenkins CI builds to detect build issues *before*
the changes get merged.

Change-Id: I44b0e611348a22b390b6ec89d3ed1d7eb7bddd63
Reviewed-on: https://gerrit.libreoffice.org/68165
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>