gnrc: replace ng_pktbuf_add() with ng_pktbuf_mark() where it's needed

sys/net/application_layer/ng_zep/ng_zep.c

sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c

sys/net/network_layer/ng_ipv6/ext/ng_ipv6_ext.c

sys/net/network_layer/ng_ipv6/ng_ipv6.c

sys/net/network_layer/ng_sixlowpan/iphc/ng_sixlowpan_iphc.c

sys/net/network_layer/ng_sixlowpan/ng_sixlowpan.c

sys/net/transport_layer/ng_udp/ng_udp.c