Don't enable the secp256k1 multiset module when building bitcoin abc

Summary:
The module is not used by the node software, and is expertimental. This
is an oversight from D5431.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5442