contrib: update ignore files for git repositories

Ignore downloaded git repos.
Signed-off-by: Rafaël Carré <funman@videolan.org>
(cherry picked from commit 96e4e4a0)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/tarballs/.gitignore

@@ -2,3 +2,4 @@
 *.zip
 *.h
 *.tgz
+*-git/
\ No newline at end of file