shared/bus-util: add missing `set.h` include

shared/bus-util: add missing `set.h` include

```
In file included from ../src/shared/bus-get-properties.c:4:
../src/shared/bus-message-util.h:22:60: error: unknown type name ‘Set’
   22 | int bus_message_append_string_set(sd_bus_message *m, const Set *s);
      |                                                            ^~~
../src/shared/bus-get-properties.c: In function ‘bus_property_get_string_set’:
../src/shared/bus-get-properties.c:178:9: error: unknown type name ‘Set’
  178 |         Set **s = ASSERT_PTR(userdata);
      |         ^~~
```

follow-up to https://github.com/systemd/systemd/commit/91080bc9733b5b2478bfc0ed58f6a7ae5da7e639