Link univalue in the seeder

Summary:
As it turns out, util depends on univalue, so it should be linked in.

The only reason it doesn't fail right now is that the linker is able to cull dead symbol that uses it before symbol resolution, which is 100% implementation defined.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5896