and fix the regex in the plugin for matching += operator

Change-Id: I26b3e3fac1d4ef3e756cc9431b983b5f27ee76d6
Reviewed-on: https://gerrit.libreoffice.org/50037Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>