drupal not detecting function in custom module's inc file
By default inc files are not included(only module files are included). They should be called explicitly like below before making a call to a function in that file.

module_load_include('inc', 'commerce_discount', 'commerce_discount.rules');


Further Reading on inc files and how they differ from .module files : https://drupal.stackexchange.com/a/54069/4876