The common good of national security is safety, which leads to a better quality of life for all of us.
A common good is something that is shared by all of the members of a society. It is something that benefits everyone in the society and cannot be taken away from one without taking it away from all.
National security allows us to live our lives without having to be afraid of attacks on us from foreigners who wish to harm us. If we are immune from such fears, we all benefit. These benefits come in the form of decreased fear and stress and increased ability to enjoy our lives. These are unquestionably good things.
National security, then, is a common good because it allows us to get more enjoyment from our lives.