fix missing USB symbols for STM32F2

When I try to compile code with `libopencm3_stm32f2` that uses USB functions I hit errors:

```
undefined reference to `usbd_ep_setup'
undefined reference to `usbd_ep_read_packet'
undefined reference to `usbd_ep_write_packet'
undefined reference to `usbd_register_control_callback'
undefined reference to `usbd_init'
undefined reference to `usbd_register_set_config_callback'
undefined reference to `usbd_poll'
```

I noticed there are some object files missing from `lib/stm32/f2/Makefile`. Fix is attached.