syscall: fix arm build

R=r
CC=golang-dev
https://golang.org/cl/4105057

src/pkg/syscall/ztypes_linux_arm.go

@@ -23,6 +23,7 @@ const (
 	SizeofSockaddrUnix      = 0x6e
 	SizeofSockaddrLinklayer = 0x14
 	SizeofLinger            = 0x8
+	SizeofIpMreq            = 0x8
 	SizeofMsghdr            = 0x1c
 	SizeofCmsghdr           = 0xc
 	SizeofUcred             = 0xc
@@ -221,6 +222,11 @@ type Iovec struct {
 	Len  uint32
 }
 
+type IpMreq struct {
+	Multiaddr [4]byte /* in_addr */
+	Interface [4]byte /* in_addr */
+}
+
 type Msghdr struct {
 	Name       *byte
 	Namelen    uint32