linux: include sys/socket.h in linux/vm_sockets.h

The header requires struct sockaddr declared. So, otherwise, we need to
include sys/socket.h earlier than linux/vm_sockets.h.
Let's make the header includable at any place.

src/basic/socket-util.h

@@ -6,12 +6,12 @@
 #include <linux/if_infiniband.h>
 #include <linux/if_packet.h>
 #include <linux/netlink.h>
-#include <sys/socket.h> /* linux/vms_sockets.h requires 'struct sockaddr' */
 #include <linux/vm_sockets.h>
 #include <netinet/in.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <string.h>
+#include <sys/socket.h>
 #include <sys/types.h>
 #include <sys/un.h>