Fix wxCompositeWindow compilation after the latest change.

SetSize() doesn't have any dependent parameters so call it via an explicit
"this->" to make it work with standard conforming compilers (and not MSVC).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77757 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775