Merge pull request #4654 from Yonezawa-T2/fix_ipv6_ext

Makefile.dep

sys/include/net/gnrc/ipv6.h

sys/include/net/gnrc/ipv6/ext.h

sys/include/net/gnrc/nettype.h

sys/include/net/gnrc/pkt.h

sys/include/net/gnrc/pktbuf.h

sys/net/gnrc/network_layer/ipv6/ext/gnrc_ipv6_ext.c

sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c

sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c

sys/net/gnrc/routing/rpl/srh/gnrc_rpl_srh.c

tests/gnrc_ipv6_ext/Makefile

tests/gnrc_ipv6_ext/README.md

tests/gnrc_ipv6_ext/main.c

tests/gnrc_ipv6_ext/tests/01-run.py