bt_field_class_*_create(): accept mandatory trace class

bt_field_class_*_create(): accept mandatory trace class

This patch makes all the field class creation functions accept a
mandatory trace class parameter. This parameter is not used for the
moment, but it could serve in the future to control allocation (for
example, object pooling), bookkeeping, and validation.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>