Script to check for duplicate includes in Kea

Already done in qa-dhcp#244 & qa-dhcp!89, but it would be nice for this to be added to tools in Kea instead, so that we can check it in Gitlab CI. More checks! 🙈 🙉 🙊

From original issue:

It would be nice to have a script that detects duplicate includes which are never, ever useful.

Unless you're including in the middle of a function or something similar which is valid C++, but not commendable C++.