vcl: include variable names in definition of OutputDevice::DrawPolyLine()

Change-Id: Ifde547b0e883871a7d30e93adbe868faa74bef71
Reviewed-on: https://gerrit.libreoffice.org/12189
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>