ninja: Let TOC code use $lib instead of ${lib}.

No functionality change, but more consistent with -MF $out.d, and the
generated files shrink by a few bytes too.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/169633003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1853 78cadc50-ecff-11dd-a971-7dbc132099af