Fix callback signature for TAP-Win32 device_handle_read().

This fixes the following compiler warning when building for Windows:

mingw/device.c: In function ‘setup_device’:
mingw/device.c:186:2: error: passing argument 2 of ‘io_add_event’ from incompatible pointer type [-Werror]
  io_add_event(&device_read_io, device_handle_read, NULL, CreateEvent(NULL, TRUE, FALSE, NULL));
  ^
In file included from mingw/../net.h:27:0,
                 from mingw/../subnet.h:24,
                 from mingw/../conf.h:34,
                 from mingw/device.c:26:
mingw/../event.h:61:13: note: expected ‘io_cb_t’ but argument is of type ‘void (*)(void *)’
 extern void io_add_event(io_t *io, io_cb_t cb, void* data, WSAEVENT event);