Fix usage for add_c_file.py

add_c_file.py was added in commit 2f31c814 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.