Changed the declaration of the variable to Marlin_main.cpp to fix issue on commit #965
Merge pull request #989 from bq/cancel_heatup

Changed the declaration of the variable "cancel_heatup" to Marlin_main.cpp to fix issue on commit #965
Merge branch 'Marlin_v1' of https://github.com/ErikZalm/Marlin into Marlin_v1