Tell git to ignore all private subdirectories

Internal builds may have private*/ subdirectories cloned from
repositories that we're not (yet) allowed to share
publicly (boo). Tell git not to complain about them every time I
run "git status".

BUG=none
BRANCH=none
TEST=manual

  mkdir private-foo
  touch private-foo/bar.txt
  git status

Observe the lack of complaints about private-foo

Change-Id: I5281e3a533f9b1a548ced6f6716a388152c58776
Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/341032
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

.gitignore

@@ -1,6 +1,5 @@
 build/
-private/
-private-cr51/
+private*/
 *.swp
 *.pyc
 tags