.gitignore: add piplib
The piplib submodule was removed all the way back
isl-0.01-134-g309036cf4f (add copyright statements,
Wed Dec 16 19:25:40 2009 +0000), initially by mistake and
then properly in
isl-0.01-146-gff1ea6be3f (properly remove
piplib submodule, Wed Jan 13 09:59:16 2010 +0100).
However, it may not get removed completely when moving from
an earlier commit to a later one since it may itself leave
some untracked files. Just ignore it in that case.
Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>