Quell warnings for exclusion list dictionaries in MSVS generator.

When using ! or / filtering mechanisms on MSVS tool dictionaries, the resulting automatically created exclusion list dictionary causes the generator to complain that the setting name is unknown. This silences these spurious warnings.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/140483003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1845 78cadc50-ecff-11dd-a971-7dbc132099af