fix openbsd problem?

CHANGES

@@ -1,6 +1,7 @@
 0.22:
   uh, the scope_id detection #defined the wrong constant.  libowfat
     always thought there was no scope_id.  Oops.
+  #include <unistd.h> in io/io_sendfile.c
 
 0.21:
   errno cleanup and man page updates (Rolf Eike Beer)

io/io_sendfile.c

@@ -114,6 +114,7 @@ int64 io_sendfile(int64 s,int64 fd,uint64 off,uint64 n) {
 #else
 
 #include <iob.h>
+#include <unistd.h>
 
 static int64 writecb(int64 s,const void* buf,uint64 n) {
   return write(s,buf,n);