build: Require vala to dist
build: Require vala to dist

Vala is needed to create the distribution tarball, otherwise
files that would be used when --enable-vala is used against the
tarball would fail because of missing files.

grilofw/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,GNOME/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,jasuarez/grilo,grilofw/grilo,grilofw/grilo,MathieuDuponchelle/grilo,GNOME/grilo