If the file is not listed, then "make dist" will not include it.
Tested: "make distcheck" works after this fix is applied.
Fixes: 139b011ab81ccea1d51f09e0261a1c390115c6ff
src/libsystemd-network/sd-icmp6-nd.c \
src/libsystemd-network/sd-dhcp6-client.c \
src/libsystemd-network/dhcp6-internal.h \
+ src/libsystemd-network/dhcp6-protocol.h \
src/libsystemd-network/dhcp6-network.c \
src/libsystemd-network/dhcp6-option.c \
src/libsystemd-network/dhcp6-lease-internal.h \