Add svn:ignore for jsoncpp/source

git-svn-id: http://src.chromium.org/svn/trunk/src/third_party/jsoncpp@106942 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
tree: 760e52727a6a4f30544c4b69204a0626819a6b1e
  1. LICENSE
  2. README.chromium
  3. jsoncpp.gyp