Bluetooth: Fix compilation for entropy_bt_hci

An application with the following config fails to link on nrf53 app
core:

```
CONFIG_BT=y
CONFIG_BT_HCI_RAW=y
CONFIG_ENTROPY_GENERATOR=y
```

This happens because `entropy_bt_hci.c` uses functions from
`hci_core.c`, which is only compiled if `BT_HCI_HOST` is selected.

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>