Fix the .gitignore for the local protocol buffer defs.

Fix the .gitignore file to ignore the nested protocol buffer repos
while not ignoring the direct protocol buffer directories.

BUG=chromium:1041872
TEST=./generate.sh

Change-Id: I28578df9412d682a3b2d424e48dffcc05725e5e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/config/+/2056124
Reviewed-by: Andrew Lamb <andrewlamb@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
Tested-by: David Burger <dburger@chromium.org>
diff --git a/.gitignore b/.gitignore
index 169ad7c..f24e103 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,5 +3,6 @@
 *_pb2.py
 *.pyc
 
-# Nested protocol buffer repos.
+# Ignore nested protocol buffer repos.
 proto/**/
+!proto/api