Fix build with old linux/netlink.h

* socketutils.c: Provide a fallback definition of NETLINK_SOCK_DIAG.

Fixes: 5c256356 ("Do not include xlat/netlink_protocols.h twice")

socketutils.c

@@ -42,6 +42,10 @@
 # define UNIX_PATH_MAX sizeof(((struct sockaddr_un *) 0)->sun_path)
 #endif
 
+#ifndef NETLINK_SOCK_DIAG
+# define NETLINK_SOCK_DIAG 4
+#endif
+
 typedef struct {
 	unsigned long inode;
 	char *details;