Added dummy glXCreateWindow/glXDestroyWindow stubs

Just like glXCreateGLXPixmap/glXDestroyGLXPixmap they don't do
anything useful (and hence are broken). Still they are needed
for compiling supertuxkart 0.8 and avoid linking problems:

    undefined reference to `glXCreateWindow'
    undefined reference to `glXDestroyWindow'

The supertuxkart game runs without any obvious problems after
this tweak.