ipv4: Move struct ipv4hdr definition to separate header

proto_ipv4.c and trie.c both define the exact same struct ipv4hdr, so
get rid of the duplication and move the definition to a separate header.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
ipv6: Move struct ipv6hdr definition to separate header

proto_ipv6.c and trie.c both define the exact same struct ipv6hdr, so
get rid of the duplication and move the definition to a separate header.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>