meson: build chardev trace files when have_block

meson: build chardev trace files when have_block

The QSD depends on chardev code, and is built when have_tools is
true. This means conditionalizing chardev trace on have_system
is wrong, we need have_block which is set have_system || have_tools.

This latent bug was historically harmless because only the spice
chardev included tracing, which wasn't built in a !have_system
scenario.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>