cpp-common/bt2: add const_cast when setting user data

cpp-common/bt2: add const_cast when setting user data

If the user data type is specified as a const type,
`static_cast<void *>` will not work.  Fix that by doing a static_cast to
`const void *` then a const_cast to `void *`.

Change-Id: Ic39c9c25da1899defe4edf8c8e800297fbf2fb2d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11799
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>