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>

src/systemc/core/sc_interface.cc

@@ -30,6 +30,7 @@
 #include "base/logging.hh"
 #include "systemc/ext/core/sc_event.hh"
 #include "systemc/ext/core/sc_interface.hh"
+#include "systemc/ext/utils/sc_report_handler.hh"
 
 namespace sc_core
 {
@@ -37,6 +38,7 @@ namespace sc_core
 const sc_event &
 sc_interface::default_event() const
 {
+    SC_REPORT_WARNING("(W116) channel doesn't have a default event", "");
     static sc_event dummy;
     return dummy;
 }