unix: add missing import in readdirent_getdirentries.go

CL 182321 missed this and the trybots didn't catch it because they
neither test darwin nor dragonfly.

Change-Id: I6093805e702d528dae9035a5acf827099e0c2eaa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/183222
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/readdirent_getdirentries.go

@@ -6,6 +6,8 @@
 
 package unix
 
+import "unsafe"
+
 // ReadDirent reads directory entries from fd and writes them into buf.
 func ReadDirent(fd int, buf []byte) (n int, err error) {
 	// Final argument is (basep *uintptr) and the syscall doesn't take nil.