cmake: configure c-ares header directory in project root (was: lib)

cmake: configure c-ares header directory in project root (was: lib)

The c-ares header directory was added to the header path within `lib`,
as opposed to every other dependency which added them in the root
`CMakeLists.txt`. Such exception is no longer necessary. This patch
aligns c-ares header setup with the rest of dependencies. And also with
autotools, which also makes no exception here.

Cherry-picked from #17705
Cherry-picked from #16973

Closes #17707