turnip: Fix GCC compiles.

turnip: Fix GCC compiles.

Apparently GCC does not consider static const variables to be
integer constants, and hence the array size and the static assert
result in compile failures.

Fixes: 4b9f967cd1a "turnip: add a more complete format table"
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>