Only call xf86platformVTProbe() when it's defined

Fixes build on non-udev systems, since XSERVER_PLATFORM_BUS is only
defined in configure.ac if $CONFIG_UDEV_KMS is true.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

hw/xfree86/common/xf86Events.c

@@ -561,8 +561,10 @@ xf86VTSwitch(void)
         for (ih = InputHandlers; ih; ih = ih->next)
             xf86EnableInputHandler(ih);
 
+#ifdef XSERVER_PLATFORM_BUS
         /* check for any new output devices */
         xf86platformVTProbe();
+#endif
 
         OsReleaseSIGIO();
     }