Put singular includes in a common variable in src/module_list.py
Trac #16805: Putting some include paths in common

I'm proposing the following patch to remove duplication in the
src/module_list.py file.

URL: http://trac.sagemath.org/16805
Reported by: Snark
Ticket author(s): Julien Puydt
Reviewer(s): Volker Braun