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.

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin