license: fix license header grammar

core/cib.c

core/lifo.c

cpu/lpc1768/atom.c

cpu/lpc1768/cpu.c

cpu/lpc1768/lpc1768-lpm.c

cpu/lpc1768/syscalls.c

cpu/sam3x8e/cpu.c

examples/default/main.c

sys/posix/fd.c

sys/posix/pnet/netinet_in.c

sys/posix/pnet/sys_socket.c

sys/posix/pthread/pthread.c

sys/posix/pthread/pthread_attr.c

sys/posix/pthread/pthread_barrier.c

sys/posix/pthread/pthread_barrierattr.c

sys/posix/pthread/pthread_mutex.c

sys/posix/pthread/pthread_once.c

sys/posix/pthread/pthread_spin.c

sys/posix/strings.c

sys/posix/unistd.c