[GTK] HTML API documentation should also be installed versioned
https://bugs.webkit.org/show_bug.cgi?id=135970

Reviewed by Philippe Normand.

* Source/PlatformGTK.cmake: Append -${WEBKITGTK_API_VERSION} to
the directory name.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172622 268f45cc-cd09-0410-ab3c-d52691b4dbfc