systemc: Add a warning to sc_interface::default_event.

systemc: Add a warning to sc_interface::default_event.

The default implementation returns a dummy event, but in the Accellera
implementation it also prints a warning. Print a warning as well, so
that the output matches for the tests.

Change-Id: I1ae2755685515c3fe538f4075af396e963cf155d
Reviewed-on: https://gem5-review.googlesource.com/c/12970
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>