Use explicit pointer cast for zopnnd call in zfiobf.c

The first argument in ZOPNND() should really be declared as a `char *`.
However it is a SPP called function and therefore limited to the SPP
data types; so the first argument is declared as `PKG_CHAR *` (which
is a `short int`). Therefore we need to cast it when called from C.

unix/os/zfiobf.c

@@ -767,7 +767,7 @@ vm_connect (void)
  "vmclient (%s): open server connection `%s' -> ",
  vm_client, osfn);
 
- ZOPNND (osfn, &acmode, &fd);
+ ZOPNND ((PKCHAR *)osfn, &acmode, &fd);
  if (fd == XERR) {
  if (vm_debug)
  fprintf (stderr, "failed\n");