[GTK] Wrong guard USE(GLX) in RedirectedXCompositeWindow
https://bugs.webkit.org/show_bug.cgi?id=112600

Reviewed by Martin Robinson.

Replace USE(GLX) condition with USE(OPENGL) && PLATFORM(X11).

* platform/gtk/RedirectedXCompositeWindow.cpp: Updated guard.
* platform/gtk/RedirectedXCompositeWindow.h: Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146196 268f45cc-cd09-0410-ab3c-d52691b4dbfc