Re-generate tracetool output when the tracetool source code changes. Use
the same approach as qapi_gen_depends and introduce a tracetool_depends
files list so meson is aware of the dependencies.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210125110958.214017-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>