how to use a submit handler which is in an .inc file
You want form_load_include() instead:

Ensures an include file is loaded whenever the form is processed.
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Use this function instead of module_load_include() from inside a form constructor or any form processing logic as it ensures that the include file is loaded whenever the form is processed. In contrast to using module_load_include() directly, form_load_include() makes sure the include file is correctly loaded also if the form is cached.