Fix pkg-config file to respect ${prefix} variable for libs and headers

Current pkg-config file does not include the ${prefix} pkg-config variable so the final result is not an absolute path but a relative one ('lib/').

Error was detected while testing sdformat using urdfdom pkg-config file, which will use console_bridge and end up showing errors [like this](https://bitbucket.org/osrf/sdformat/pull-request/77/support-to-use-external-urdf/diff#comment-899144). Fixing the console bridge pkg-config manually, with this pull request changes, fix the issue for me.

I believe that was @thomas-moulard who introduced the changes to the pkg config file.