lib: Explicit cast to bt_plugin_find_status

lib: Explicit cast to bt_plugin_find_status

Clang 8.0:
   error: implicit conversion from enumeration type 'bt_plugin_find_all_status' (aka 'enum bt_plugin_find_all_status') to different enumeration type 'enum bt_plugin_find_status' [-Werror,-Wenum-conversion

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: Ie8304eccaa329b953837eb24e5fe4c2b2b81f320
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1765
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>