Fix usage for add_c_file.py
add_c_file.py was added in commit 2f31c8146f in 0.4.1.2-alpha.
But it has a bug: the suggested usage fails with:
$ scripts/maint/add_c_file.py ./src/feature/dirauth/ocelot.c Made files successfully but couldn't identify include.am for ./src/feature/dirauth/ocelot.c Exit 1
The correct usage has no "./":
$ scripts/maint/add_c_file.py src/feature/dirauth/ocelot.c
We should fix the usage, or make topdir_file() use python's canonical path functions.