gst-validate meson support missing from release tarball

Submitted by Jan Tojnar @jtojnar

Link to original bug (#794871)

Description

meson.build is not listed in EXTRA_DIST causing the file to be omitted from release tarball.

https://github.com/GStreamer/gst-devtools/blob/1.14.0/validate/Makefile.am

Version: 1.14.0