Adding missing include in svg/GradientAttributes.h

If you managed to include GradientAttributes.h before you included
SVGGradientElement.h you would have no definition of
SVGSpreadMethod and other types.

R=fs@opera.com

Review-Url: https://codereview.chromium.org/2790623002
Cr-Commit-Position: refs/heads/master@{#460791}
1 file changed