py: Fix mp_obj_t -> mp_const_obj_t for mp_obj_int_get_checked()

@pfalcon: this fixes the following build error introduced in commit ab7bf28489fde83c73da2a3a20b9eadb08e8cc81

../py/objint.c:261:15: error: conflicting types for ‘mp_obj_int_get_checked’
In file included from ../py/objint.c:36:0:
../py/obj.h:455:15: note: previous declaration of ‘mp_obj_int_get_checked’ was here