kern linker: Make linker_file_add_dependency() void

The only possible return value has been zero since cee9542d51f0.

No functional change intended.

Reviewed by: dfr
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44507

(cherry picked from commit 1c7307cf6763a295ea9896be9f197d40511c77ed)