Add a message to static_assert
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

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