Update shared library version to indicated that new APIs were added

This pull request update the libtool versioning data in configure.ac for the libusb shared library.

There is a nice comment in your configure.ac that explains how to properly do this, unfortunately, it seems this has been ignored: Many new APIs have been added to libusb and old ones extended, without the shared library version reflecting this. As a result, users can potentially get into a broken state if libusb(x) is downgraded.

This commit does the minimal work necessary for the next release. Of course, this problem will come back whenever a new API is added. Hence, ideally in the future, the libtool current/revision/age values should be checked and (if necessary) updated before every release.