cf390df build: silence gcc7's implicit fallthrough warning (Cory Fields)
Tree-SHA512: 330a4258161529c6e7af34ba3dc9703c24d897d7fff317c078441405c4546c7236e260603181001954b214d4640ad09ed3d34d3b518be991cb1712a02bcaa1d9
Backport of Core PR 10489
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.