doc: Add since version tag to libusb_set_log_cb()

The commit f74c7f5 "core: new API libusb_set_log_cb() to redirect
global and per context log messages to the provided log handling
function" in 2018-12-05 is present from 1.0.23 onwards.

This commit just adds this to the docs.

Closes #872

Signed-off-by: Victor Toso <victortoso@redhat.com>
Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
3 files changed