Avoid compiler warning for unused glx_dpy variable.

My earlier attempt to eliminate this warning (c0ca2bfb2a) was
invalid as it removed the variable declaration. Jerome correctly
reverted that (600c85efdb) since the variable is used when
X_DRI2SwapBuffers is defined.

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

src/glx/dri2.c

@@ -88,7 +88,6 @@ static Bool
 DRI2WireToEvent(Display *dpy, XEvent *event, xEvent *wire)
 {
    XExtDisplayInfo *info = DRI2FindDisplay(dpy);
-   __GLXdisplayPrivate *glx_dpy = __glXInitialize(dpy);
 
    XextCheckExtension(dpy, info, dri2ExtensionName, False);
 
@@ -100,6 +99,7 @@ DRI2WireToEvent(Display *dpy, XEvent *event, xEvent *wire)
       GLXBufferSwapComplete *aevent = (GLXBufferSwapComplete *)event;
       xDRI2BufferSwapComplete *awire = (xDRI2BufferSwapComplete *)wire;
       __GLXDRIdrawable *pdraw;
+      __GLXdisplayPrivate *glx_dpy = __glXInitialize(dpy);
 
       /* Ignore swap events if we're not looking for them */
       pdraw = dri2GetGlxDrawableFromXDrawableId(dpy, awire->drawable);