Forbid server grabs by non-WM on *rootless* XWayland

a77d95af intended to do this, but the
check for “is this rootless or rootful XWayland” was inverted.

Fixes: a77d95af ("xwayland: Prevent Xserver grabs with rootless")
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>

hw/xwayland/xwayland-screen.c

@@ -685,7 +685,7 @@ static void
 xwl_screen_setup_custom_vector(struct xwl_screen *xwl_screen)
 {
     /* Rootfull Xwayland does not need a custom ProcVector (yet?) */
-    if (xwl_screen->rootless)
+    if (!xwl_screen->rootless)
         return;
 
     xwl_screen->GrabServer = ProcVector[X_GrabServer];