extrakeys: Init empty experimental module

Summary:
This is to prepare for xonly_pubkeys and keypairs.

This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/47e6618e11813cfabe91f0909ca031f960cb7dd4

Test Plan:

ninja check-secp256k1

And with the module on:

cmake -GNinja .. -DSECP256K1_ENABLE_MODULE_EXTRAKEYS=On
ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7638