GN: Add BUILD.gn file to third_party/jsoncpp

BUG=webrtc:3441
TEST=Successful build with WebRTC, see
https://review.webrtc.org/17669004/ for details.

Review URL: https://codereview.chromium.org/322373002

git-svn-id: http://src.chromium.org/svn/trunk/src/third_party/jsoncpp@276676 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed