tnef: fix `gcc-15` build failure

Without the change the build fails on `master` as
https://hydra.nixos.org/build/318200761:

```
gcc -DHAVE_CONFIG_H -I.     -g -O2 -c -o attr.o attr.c
attr.c:242:1: error: conflicting types for 'attr_read'; have 'Attr *(FILE *)'
  242 | attr_read (FILE* in)
      | ^~~~~~~~~
In file included from attr.c:29:
attr.h:72:14: note: previous declaration of 'attr_read' with type 'Attr *(void)'
   72 | extern Attr* attr_read ();
      |              ^~~~~~~~~
```

0 files changed, 0 insertions, 0 deletions