doc: fix a typo in adding-syscalls.rst

There was a typo in adding-syscalls.rst that could mislead developers
to add a C filename in a makefile instead of an object filename.
This error, while not keeping developers from contributing could slow
the development process down by introducing build errors.

Signed-off-by: Guillaume Dore <corwin@poussif.eu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>