unix: add solaris to mmap test

Fixes golang/go#12255.

Change-Id: Ie920d655de2531e9ddbd0bedcf34eaa8c9cace5a
Reviewed-on: https://go-review.googlesource.com/13816
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mmap_unix_test.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 netbsd openbsd
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package unix_test