x/net/ipv6: add sticky source-specific multicast socket options

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/175010043
diff --git a/ipv6/sys_bsd.go b/ipv6/sys_bsd.go
index cc5bc5c..5e10381 100644
--- a/ipv6/sys_bsd.go
+++ b/ipv6/sys_bsd.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 dragonfly freebsd netbsd openbsd
+// +build dragonfly netbsd openbsd
 
 package ipv6