unix: disable sysctl on iOS

Running the regenerating scripts also brought in ClockGettime.

Updates golang/go#34133

Change-Id: I0eb9ed6dbbc2bdd7e3d2a7f5d88492e9dfed0ada
Reviewed-on: https://go-review.googlesource.com/c/sys/+/194097
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_dragonfly.go

@@ -14,6 +14,8 @@ package unix
 
 import "unsafe"
 
+//sys	sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS___SYSCTL
+
 // SockaddrDatalink implements the Sockaddr interface for AF_LINK type sockets.
 type SockaddrDatalink struct {
 	Len    uint8