fix: visibility of bt_plugin_python_create_all_from_file

fix: visibility of bt_plugin_python_create_all_from_file

With glib2 < 2.52, G_MODULE_EXPORT is defined to nothing on platforms
other than Windows instead of __attribute__((visibility("default")))
which results in an hidden symbol and the failure to load the python
plugin provider.

Use our own macro to export this symbol.

Change-Id: I83ac515c573d92e22021759bf23a1a9615bd3e07
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9724
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>