package/protobuf-c: threads dependency is a direct dependency

Currently, protobuf-c's dependency on threads is labelled as being
inherited from protobuf.

This is wrong, as protobuf-c does not depend on protobuf, and such
dependency was removed in e16865a5 (protobuf-c: Don't require protobuf on
target), but forgot to remove the corresponding comment.

Remove it now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>