It was incorrectly looking for a file in src/libsystemd-network/ when the file was actually deployed to src/systemd/ instead. This broke "make dist".
Tested: "make dist" works again after this patchset is applied.
Fixes: f20a35cc0d537dd4cfc1054cf7936b04a1700f3a
test_icmp6_rs_SOURCES = \
src/systemd/sd-dhcp6-client.h \
- src/libsystemd-network/sd-icmp6-nd.h \
+ src/systemd/sd-icmp6-nd.h \
src/libsystemd-network/dhcp6-internal.h \
src/libsystemd-network/test-icmp6-rs.c