schnorrsig: Init empty experimental module

Summary: This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/7a703fd97db0161bae07ef84513ddde6e0d27353

Test Plan:

ninja check-secp256k1

With missing extrakeys:

cmake -GNinja .. -DSECP256K1_ENABLE_MODULE_SCHNORRSIG=On

Check that we get an error.

And with the module:

cmake -GNinja .. -DSECP256K1_ENABLE_MODULE_EXTRAKEYS=On -DSECP256K1_ENABLE_MODULE_SCHNORRSIG=On
ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7646