gst-devtools: add version 1.18.0 (gst-validate -> gst-devtools)

The gst-validate tarball has been superseded by the gst-devtools tarball for consistency with the git module name.

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>