contrib: don't ignore triplets in subfolders

Only ignores triplets in the root contrib path. Previous
implementation had the potential to ignore triplets in the
subfolders.

This will allow for creating:
1) Patches using this format (e.g. contrib/src/*/track-me-now.patch).
2) Subfolders beneath contrib's root (but not in contrib itself)
(e.g. contrib/src/track-me-now/*).
OK-ed-by: Rafaël Carré <funman@videolan.org>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/.gitignore

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