pkg-config usage in kea libs is wrong

1. The file should be named kea-dns++.pc and not dns++
2. It references b10 libraries: -lb10-dns++ -lb10-cryptolink -lb10-util -lb10-exceptions
3. It references -lm which is probably not needed (I am guessing kea-dns++ doesn't export any of it's symbols)

It's also quite confusing that this is the only pkg-config file and it's only dns++. You should probably have accompanying .pc file (with correct intra-library dependencies declared) for every kea library.