lib: add bt_clock_class_get_graph_mip_version()

lib: add bt_clock_class_get_graph_mip_version()

Add bt_clock_class_get_graph_mip_version, which returns the MIP version
for the graph the clock class is in, for the same reason as explained in
fbc90342dade ("lib: add bt_trace_class_get_graph_mip_version") for trace
class.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I6949c47e39b2c4dfc8e0d55ab701e6eef9fb0d45
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13394
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>