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 systemd/systemd@91080bc

(cherry picked from commit ca97d48bd425685d4ec20397812c3b1b006f5070)

Resolves: RHEL-73780

src/shared/bus-message-util.h

@@ -4,6 +4,7 @@
 #include "sd-bus.h"
 
 #include "in-addr-util.h"
+#include "set.h"
 #include "socket-netlink.h"
 
 int bus_message_read_id128(sd_bus_message *m, sd_id128_t *ret);