os/user: fix osusergo build on Solaris

Verified that on on Linux, with:

CGO_ENABLED=1 GOOS=solaris go install --tags=osusergo

... it builds now.

Updates #24841
Updates #24845

Change-Id: I49f40532bc2a13a9d282771592fc8d7f116b1902
Reviewed-on: https://go-review.googlesource.com/107304Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/os/user/listgroups_solaris.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 cgo
+// +build cgo,!osusergo
 
 // Even though this file requires no C, it is used to provide a
 // listGroup stub because all the other Solaris calls work.  Otherwise,