x/sys/unix: remove nacl build tag from dirent code

[smasher164]

CL 128110043 deletes nacl from x/sys/unix, but a later change CL 38758 copies stdlib syscall code over for non-nacl platforms. However, x/unix/dirent.go#L5 still includes +build nacl, which should be removed.

[gopherbot]

Change https://golang.org/cl/188361 mentions this issue: x/sys/unix: remove nacl build tag from dirent code