Server: fix use after free in deinit()

The input manager and seats must be destroyed before the display is
destroyed as they need to destroy their timer event sources used for key
repeat.

river/Server.zig

@@ -144,10 +144,10 @@ pub fn deinit(self: *Self) void {
     self.backend.destroy();
 
     self.root.deinit();
+    self.input_manager.deinit();
 
     self.wl_server.destroy();
 
-    self.input_manager.deinit();
     self.config.deinit();
 }