gm: Include system_preferences.h in mac file.

This fixes the FIXME in there.

R=epoger@google.com

Review URL: https://codereview.appspot.com/6576049

git-svn-id: http://skia.googlecode.com/svn/trunk@5692 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed