Merge #10489: build: silence gcc7's implicit fallthrough warning

Summary:
cf390df build: silence gcc7's implicit fallthrough warning (Cory Fields)

Tree-SHA512: 330a4258161529c6e7af34ba3dc9703c24d897d7fff317c078441405c4546c7236e260603181001954b214d4640ad09ed3d34d3b518be991cb1712a02bcaa1d9

Backport of Core PR10489
https://github.com/bitcoin/bitcoin/pull/10489/files
I noticed these fallthrough warnings appearing repeatedly due to tinyformat as the original PR describes.
Although I don't 100% agree that it is good to always silence this warning, it is proving to add
cognitive load when digging through real build error messages. I'll leave this diff up for discussion.

Test Plan:

make # no longer vomits warnings related to fallthrough
ninja # does not show fallthrough warnings

cmake -GNinja -DEXTRA_WARNINGS=1 ..
ninja # vomits fallthrough warnings as expected

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2960