Fix: bt2: Make bindings target depend on convenience libraries

Fix: bt2: Make bindings target depend on convenience libraries

Three convenience libraries are used to build the bindings' native
library.  This dependency is not expressed in the Makefile, so if you
change one of them and re-run make, the bindings won't be rebuilt, and
the changes in that convenience lib won't be taken into account.  For
example, if you touch src/py-common/.libs/libbabeltrace2-py-common.a and
re-run make, the bindings should (but currently don't) get re-built.

This patch adds the explicit dependency in the Makefile, so that any
change to one of these libs will trigger a rebuild of the bindings.

Change-Id: Iea47833b206bcd0f1cc0d3f72975b14fd6eded93
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1736
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>