WIP: Create aliases for old versioned symbols

I would recommend waiting to see what upstream say about #33 before applying anything like this in Debian - if upstream reject it, even if we decide that keeping ABI is more important than avoiding this delta, that should be a decision and not the default.

• New upstream version 0.3.4

  We need this before messing with versioned symbols, because upstream have already made some changes in this area.

• Revert accidental removal of g_usb_source_set_callback from the ABI

  Proposed upstream at https://github.com/hughsie/libgusb/pull/31

• Generate backwards-compatible versioned symbols

  As requested by @jcristau on #951209 as an alternative to what @bigon proposes (see 696adae5). Proposed upstream at https://github.com/hughsie/libgusb/pull/33