scons: Make the sim_objects parameter of SimObject mandantory.

If there really are no c++ sim_objects in the file, then sim_objects can
be set to [] which it used to default to.

This way, if someone hasn't remembered to update their SConscript files
for the new sim_objects and enums parameters, this will give them some
indication what's wrong, rather than the build just failing later.

Change-Id: Ic1933f7b9dfff7dd7e403c6c84f1f510c8ee8c72
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54203
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/cpu/o3/SConscript

@@ -32,7 +32,7 @@ Import('*')
 
 if 'O3CPU' in env['CPU_MODELS']:
     SimObject('FUPool.py', sim_objects=['FUPool'])
-    SimObject('FuncUnitConfig.py')
+    SimObject('FuncUnitConfig.py', sim_objects=[])
     SimObject('O3CPU.py', sim_objects=['O3CPU'], enums=[
         'SMTFetchPolicy', 'SMTQueuePolicy', 'CommitPolicy'])