Don't exclude sources for 'none' targets

MSVS doesn't compile files for 'none' targets, so there is no need to "exclude"
them.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/282843002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1920 78cadc50-ecff-11dd-a971-7dbc132099af