Avoid compiler warning for unused glx_dpy variable.

Avoid compiler warning for unused glx_dpy variable.

My earlier attempt to eliminate this warning (c0ca2bfb2ad8c) was
invalid as it removed the variable declaration. Jerome correctly
reverted that (600c85efdb0ff) since the variable is used when
X_DRI2SwapBuffers is defined.

Here, instead of removing the declaration, we move it to inside the
correct #ifdef.