winex11.drv: Make sure that the selectionAcquired flag has been set before returning from X11DRV_AcquireClipboard.

X11DRV_CLIPBOARD_UpdateCache depends on selectionAcquired being set if
the current process is the selection owner, otherwise it will defer to
getting the clipformats from X, manufacturing extra formats that the
app may not be expecting, having just set the formats itself. Worse
still, since selectionAcquired is set in another thread this behaviour
is not predicatable and it may sometimes use the clipformats already
set and other times defer to X.