Merge r173999 - [GTK] Enable CSS_IMAGE_SET in production builds
https://bugs.webkit.org/show_bug.cgi?id=137142

Reviewed by Alejandro G. Castro.

This is required by the inspector to show some of the icons that
has a HiDPI variant.

* Source/cmake/OptionsGTK.cmake:

git-svn-id: http://svn.webkit.org/repository/webkit/releases/WebKitGTK/webkit-2.6@175215 268f45cc-cd09-0410-ab3c-d52691b4dbfc