systemc: Add a stub kernel SimObject.

The guts of this class will be added in later changes.

Change-Id: I3582c40f88f7d9ba6028a6f0a8ee5c32924a65bf
Reviewed-on: https://gem5-review.googlesource.com/10822
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/systemc/SConscript

@@ -30,4 +30,6 @@ Import('*')
 if env['USE_SYSTEMC']:
     SimObject('SystemC.py')
 
+    Source('kernel.cc')
+
     Source('sc_object.cc')