Don't report Dynamic inheritance warning on Dynamic itself

[allanrenucci]

Compiling the standard library, we used to get this warning:

-- Feature Warning: Test.scala:3:6 ---------------------------------
3 |trait Dynamic extends Any
  |^^^^^^^^^^^^^^^^^^^^^^^^^
  |extension of type scala.Dynamic should be enabled
  |by making the implicit value scala.language.dynamics visible.
  |This can be achieved by adding the import clause 'import scala.language.dynamics'
  |or by setting the compiler option -language:dynamics.
  |See the Scala docs for value scala.language.dynamics for a discussion
  |why the feature should be explicitly enabled.

[liufengyun]

LGTM