Building sscan R2-12 fails with an error on Ubuntu 26

gcc 15.2.0 (Ubuntu 26) reports an error:

```
writeXDR.c:165:24: error: too many arguments to function 'xdr_elem'; expected 0, have 2
```

which is caused by the fact that `xdrproc_t` is declared as:

```
typedef int (*xdrproc_t)();
```

while the function `xdrproc_t xdr_elem` is called as:

```
if (! (*xdr_elem)(fd, elptr)) {
```
