mesa: Add missing build dep

Mesa uses python-packaging to check whether mako is installed, and fails
to find mako if packaging isn't available (e.g., in trace mode).

No revbump, since it either builds correctly, or not at all.

x11/mesa/Portfile

@@ -41,6 +41,7 @@ depends_build-append    path:bin/pkg-config:pkgconfig \
                         port:bison \
                         port:python${py_ver_nodot} \
                         port:py${py_ver_nodot}-mako \
+                        port:py${py_ver_nodot}-packaging \
                         port:py${py_ver_nodot}-yaml \
                         port:xorg-xorgproto