mem: Add "const" attribute to Packet getters
mem: Add "const" attribute to Packet getters

Add a "const" keywords to the getters in the Packet class so these can be
invoked on const Packet objects.

haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5