[SECP256K1] Fix Travis missing ninja

Summary: The ninja package was mistakenly removed in D5524.

Test Plan:
Run the Travis build (see
https://travis-ci.org/github/Fabcien/secp256k1/builds/667699591).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5588