inet_csum: remove ng_ prefix

Makefile.dep

sys/Makefile

sys/include/net/inet_csum.h

sys/include/net/ng_ipv6/hdr.h

sys/net/crosslayer/inet_csum/Makefile

sys/net/crosslayer/inet_csum/inet_csum.c

sys/net/network_layer/ng_icmpv6/ng_icmpv6.c

sys/net/transport_layer/ng_udp/ng_udp.c

tests/unittests/tests-inet_csum/Makefile.include

tests/unittests/tests-inet_csum/tests-inet_csum.c

tests/unittests/tests-ipv6_hdr/tests-ipv6_hdr.c