res_parking: Add parking_devicestate.c left out from previous commit

(issue ASTERISK-21645)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2545/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390920 65c4cc65-6c06-0410-ace0-fbb531ad65f3