sys: add missig <string.h> includes

sys/net/application_layer/gcoap/gcoap.c

sys/net/gnrc/netif/gnrc_netif.c

sys/net/gnrc/netif/gnrc_netif_ethernet.c

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h

sys/net/gnrc/network_layer/ndp/gnrc_ndp.c

sys/net/gnrc/network_layer/sixlowpan/nd/gnrc_sixlowpan_nd.c

sys/net/gnrc/routing/rpl/gnrc_rpl.c

sys/net/gnrc/routing/rpl/gnrc_rpl_control_messages.c

sys/net/gnrc/routing/rpl/gnrc_rpl_dodag.c

sys/net/gnrc/routing/rpl/p2p/gnrc_rpl_p2p.c

sys/net/gnrc/sock/ip/gnrc_sock_ip.c

sys/net/gnrc/sock/udp/gnrc_sock_udp.c

sys/net/gnrc/transport_layer/tcp/gnrc_tcp.c

sys/net/routing/nhdp/iib_table.c

sys/net/routing/nhdp/nhdp.c

sys/random/fortuna.c

sys/random/fortuna/fortuna.c

sys/shell/commands/sc_ccnl.c

sys/shell/commands/sc_gnrc_netif.c