os/signal/internal/pty: build on Solaris

    
    Patch from Rainer Orth.
    
    Reviewed-on: https://go-review.googlesource.com/87037

From-SVN: r256399

gcc/go/gofrontend/MERGE

-29e821cf865aa6ee06cee9dae9823295202b1a61
+cd422bacf0505e9656661d97a571668ad1bde0fe
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/os/signal/internal/pty/pty.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,!android netbsd openbsd
+// +build darwin dragonfly freebsd linux,!android netbsd openbsd solaris
 
 // Package pty is a simple pseudo-terminal package for Unix systems,
 // implemented by calling C functions via cgo.