Correct order of libs returned on pkg-config --libs --static libsecp256k1 call.

Summary: This is a backport of secp256k1's PR567

Test Plan:

make -j15 check

Not sure how to test the package itself, but at least nothing's broken

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4979