gyp: only include windows *_x64 targets if GYP_GENERATORS includes ninja

BUG=skia:2442
R=caryclark@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/247143003

git-svn-id: http://skia.googlecode.com/svn/trunk/gyp@14313 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed