Clean up include dirs

Move include of BLKID_INCLUDE_DIRS into scope it is used.
UUID_INCLUDE_DIRS is unused, left over from before partitionmanager
switched away from libuuid in 26e7f9d7ef306d61380e1c8965feb83bb6b07d18 .
Explicit Qt5Core_INCLUDE_DIRS no longer needed.
src/ is automatically available for kpmcore target, also exposed in
its build link interface.