term: add aix and zos build tags

Some packages depending on golang.org/x/term still need to build with
old Go versions and adding these build tags allows them build this
package with Go 1.11 and older.

Change-Id: I82d8c71120ff82d8ace07f96636f16c86edb15e5
Reviewed-on: https://go-review.googlesource.com/c/term/+/312249
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser

term_unix_aix.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build aix
+// +build aix
+
 package term
 
 import "golang.org/x/sys/unix"

term_unix_zos.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build zos
+// +build zos
+
 package term
 
 import "golang.org/x/sys/unix"