unix: add Fstatat on NetBSD

Change-Id: Id2ae518a5f6abfdae56398a3ef7f205267057257
Reviewed-on: https://go-review.googlesource.com/100616
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_netbsd.go

@@ -240,6 +240,7 @@ func Uname(uname *Utsname) error {
 //sys	Flock(fd int, how int) (err error)
 //sys	Fpathconf(fd int, name int) (val int, err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
+//sys	Fstatat(fd int, path string, stat *Stat_t, flags int) (err error)
 //sys	Fsync(fd int) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)