Add a message to static_assert

Summary:
static_assert with no message is a c++17 feature, and causes clang to
generate a -Wc++17-extensions warning.

Test Plan:
With Clang:

make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Mengerian, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox, Mengerian, markblundeberg

Subscribers: Mengerian, markblundeberg, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3794