POM: mark lwf-stubs as provided scope

The difference from compile scope is that provided is not inherited
transitively, but instead assumed to be provided by the environment.

In the case of the LuraWave support, we do not want to inflict
lwf-stubs as a dependency on any downstream code; rather, we catch
the ClassNotFoundException etc. when the LWF classes are not present.

Also, note that due to the change in scope, it is no longer necessary
to declare lwf-stubs as an exclusion in the assemblies, either.