cmd/getgo: fix build on aix

Change-Id: Ic4a2f8f32258b14354658af5a09c736404d38808
Reviewed-on: https://go-review.googlesource.com/c/tools/+/170961
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/cmd/getgo/system_unix.go b/cmd/getgo/system_unix.go
index d2405c5..adc3e55 100644
--- a/cmd/getgo/system_unix.go
+++ b/cmd/getgo/system_unix.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris
+// +build aix darwin dragonfly freebsd linux nacl netbsd openbsd solaris
 
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