contrib: ignore triplets

Ignore the triplet subfolders created in the contrib path.
Signed-off-by: Rafaël Carré <funman@videolan.org>
(cherry picked from commit ed8d730d)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/.gitignore

 vlc-contrib-*.tar.xz
+*-*-*