os/user: fix build on darwin with "osusergo" build tag

Fixes #24841
Updates #24845

Change-Id: Ia7e2deefe64c12ee8a76ce6ed9f9e003e912b161
Reviewed-on: https://go-review.googlesource.com/107299Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/user/getgrouplist_darwin.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build cgo,!osusergo
+
 package user
 
 /*