From https://github.com/stefanha/linux vsock-extras
f442d29db40a1999d139dccf110f3b590a0ed5b8:nc-vsock.c

including include/uapi/linux/vm_sockets.h, patched with:

--- /home/ijc/x	2016-03-17 09:31:23.288944691 +0000
+++ alpine/packages/nc-vsock/include/uapi/linux/vm_sockets.h	2016-03-17 09:30:07.916069307 +0000
@@ -16,7 +16,12 @@
 #ifndef _UAPI_VM_SOCKETS_H
 #define _UAPI_VM_SOCKETS_H
 
+#ifdef __KERNEL__
 #include <linux/socket.h>
+#else
+#define __kernel_sa_family_t sa_family_t
+#include <sys/socket.h>
+#endif
 
 /* Option name for STREAM socket buffer size.  Use as the option name in
  * setsockopt(3) or getsockopt(3) to set or get an unsigned long long that