dhcp6: don't include internal header "sparse-endian.h" in "sd-dhcp6-client.h"
Arguably, libsystemd-network is (still) entirely internal API.
However there is the aim of maybe exposing it as public API.
For that reason, it cannot include internal headers from
"src/basic/".
Note how files "src/systemd/sd-*.h" don't include any systemd
headers which don't themself have an "sd-" prefix.