pkg-config-wrapper should not be run when build_nss is disabled (=0).

BUG=b/15411893
R=wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/12839004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6538 4adac7df-926f-26a2-2b94-8c16560cd09d