Build fix for gtk2-based library.

Reviewed by Martin Robinson.

* platform/gtk/WidgetRenderingContext.cpp: move gtk2 check to after the config.h
include, which is the one that includes autotoolsconfig.h, where the gtk2 define
lives.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143973 268f45cc-cd09-0410-ab3c-d52691b4dbfc