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: http://svn.asterisk.org/svn/asterisk/trunk@390920 f38db490-d61c-443f-a65b-d21fe96a405b