[SECP256K1] Request --enable-experimental for the multiset module

Summary:
The module is advertised as experimental, but is missing the check for
--enable-experimental like the other experimental modules.
Also disable it by default with cmake (which is the autotools behavior),
as it is more in accordance with its experimental status.

Depends on D5429.

Test Plan:

../configure --enable-module-multiset

Check it fails with a message asking for --enable-experimental.

../configure --enable-module-multiset --enable-experimental
make check

cmake -GNinja .. -DSECP256K1_ENABLE_MODULE_MULTISET=ON
ninja check-secp256k1

cmake -GNinja ..
ninja check-secp256k1

Run the Travis build (see https://travis-ci.org/Fabcien/secp256k1/builds/658346030).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5431