[SECP256K1] Fix the Travis build wrong targets names

Summary: As per title.

Test Plan:
Run the Travis build (the failure is no longer related to target names):
https://travis-ci.org/github/Bitcoin-ABC/secp256k1/builds/702808317

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6853