[Bug c/106929] declaration in switch-case doesn't fail since GCC 11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106929

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
For C17 (and previous standards), if you use -pedantic you'll get a warning,
and -pedantic-errors will make it an error.