Always include <sys/socket.h> before <linux/netlink.h>.

Newer kernel headers files aren't stand alone. (This has long been true of
glibc's copy, but we only recently updated external/kernel-headers.)

Change-Id: Iebcb40902b2f29fe509e207e2f3a9d02a7e40602
diff --git a/libnl_2/msg.c b/libnl_2/msg.c
index 283da6e..1303e8a 100644
--- a/libnl_2/msg.c
+++ b/libnl_2/msg.c
@@ -18,6 +18,7 @@
 
 #include <malloc.h>
 #include <unistd.h>
+#include <sys/socket.h>
 #include <linux/netlink.h>
 #include "netlink-types.h"