net: fou: rename the source for linking

We'll need to link two objects together to form the fou module.
This means the source can't be called fou, the build system expects
fou.o to be the combined object.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>