libusb: Add an XXX comment for possible duplicate callback IDsHEADmain

If next_callback_id wraps we could end up with two callbacks with the
same ID.  I recommitted the original change despite this issue in order
to fix the libusb API as soon as possible after SHLIB_MAJOR was bumped
in commit 527a82474cb3 (libusb: versioning symbols).

It's very unlikely in practice that software will register and
deregister a sufficient number of callbacks to trigger this, but it is
a real issue to be fixed in a subsequent commit.

Sponsored by:	The FreeBSD Foundation

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