portlist: adjust build tags for iOS + Go 1.16

Updates #943
Updates #1370

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

portlist/netstat_exec.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build windows freebsd openbsd darwin,amd64 go1.16,darwin,arm64
+// +build windows freebsd openbsd darwin,go1.16 darwin,!go1.16,!arm64
+// +build !ios
 
 package portlist
 

portlist/portlist_macos.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin,amd64 go1.16,darwin,arm64
+// +build darwin,amd64,!go1.16 darwin,go1.16
+// +build !ios
 
 package portlist