gfe-relnote: n/a(code clean up) Add default definition of copy assignment for QuicPacketHeader. This is to make QUICHE code compatible in GCC 9.2. Error was reported here: https://github.com/envoyproxy/envoy/issues/9437 PiperOrigin-RevId: 286631852 Change-Id: I15c645399078f0be81c2331d591d6f8975b9fc55