Fix the build with GCC < 8

Summary:
Fix the `sorry, unimplemented: non-trivial designated initializers not
supported` failure with GCC version < 8.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55606 and
https://stackoverflow.com/questions/31215971/non-trivial-designated-initializers-not-supported.

Test Plan: Build with GCC 7 and check the error is gone.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5149