unix: add missing import to syscall_hurd.go

CL 471119 added a reference to the unsafe package,
but didn't add the import.

Change-Id: Ibb8097df0444d8e83ba08e4ca2b5df98844fccf4
Reviewed-on: https://go-review.googlesource.com/c/sys/+/607976
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/syscall_hurd.go

@@ -11,6 +11,7 @@ package unix
 int ioctl(int, unsigned long int, uintptr_t);
 */
 import "C"
+import "unsafe"
 
 func ioctl(fd int, req uint, arg uintptr) (err error) {
 	r0, er := C.ioctl(C.int(fd), C.ulong(req), C.uintptr_t(arg))