[secp256k1] Update ax_jni_include_dir.m4 macro

Summary: This is a backport of PR460 from Karl-Johan Alm

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1260