Use -Wno-error=suggest-attribute=format when using -Werror

Works around build failures on Debian due to a spurious 'warning' similar to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28492
1 job for master in 8 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #860
make deb!

00:08:42