os/user: fix Solaris declaration.
os/user: fix Solaris declaration.

Patch from Rainer Orth.

Change-Id: I68567a8bc86d8a26a6a62c036419007e09c85574
Reviewed-on: https://go-review.googlesource.com/25210
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend