st/wgl: Don't export wglGetExtensionsStringARB.
It's not exported by the official opengl32.dll neither. Applications are supposed to get it via wglGetProcAddress(), not GetProcAddress(). Cc: "10.4" <mesa-stable@lists.freedesktop.org> Reviewed-by:Brian Paul <brianp@vmware.com> Reviewed-by:
Roland Scheidegger <sroland@vmware.com> (cherry picked from commit cb009bdd)