[SECP256K1] Add missing secp256k1_ge_set_gej_var decl.

Summary: Backport of secp256k1#877

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9386