syscall: add if_announce support for freebsd

Update #4866.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7398047
diff --git a/src/pkg/syscall/zerrors_freebsd_arm.go b/src/pkg/syscall/zerrors_freebsd_arm.go
index b02a84c..92cade2 100644
--- a/src/pkg/syscall/zerrors_freebsd_arm.go
+++ b/src/pkg/syscall/zerrors_freebsd_arm.go
@@ -466,6 +466,8 @@
 	ICANON                            = 0x100
 	ICRNL                             = 0x100
 	IEXTEN                            = 0x400
+	IFAN_ARRIVAL                      = 0x0
+	IFAN_DEPARTURE                    = 0x1
 	IFF_ALLMULTI                      = 0x200
 	IFF_ALTPHYS                       = 0x4000
 	IFF_BROADCAST                     = 0x2