r45948 snapshot

Fix missing prototype for `usbsid_available()`

In case USBSID isn't available we'll still need the prototype for the
`usbsid_available()` stub.


git-svn-id: https://svn.code.sf.net/p/vice-emu/code/trunk@45948 379a1393-f5fb-40a0-bcee-ef074d9b53f7