missing.h: add AF_VSOCK bits

missing.h: add AF_VSOCK bits

Ubuntu 14.04 (Trusty) kernel header packages ship without
<linux/vm_sockets.h>.  Only struct sockaddr_vm and VMADDR_CID_ANY will
be needed by systemd and they are simple enough to go in missing.h.

CentOS 7 <sys/socket.h> does not define AF_VSOCK.  Define it so the code
can compile although actual socket(2) calls may fail at runtime if the
address family isn't available.