Something has changed recently which causes this error to now be emitted
when building on Windows msys2-mingw32:
```
../gio/gwin32networkmonitor.c: In function 'win_network_monitor_get_ip_info':
../gio/gwin32networkmonitor.c:92:15: error: storing the address of local variable 'prefix' in '*dest' [-Werror=dangling-pointer=]
   92 |         *dest = (guint8 *) &prefix.Prefix.Ipv4.sin_addr;
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

If `IP_ADDRESS_PREFIX` is defined as a scalar rather than a pointer,
that could explain the problem.

Change the function to always operate on a pointer to avoid any
potential such issues.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>