build opts: add MI_example to NULL ISA

A later changeset changes the file src/python/swig/pyobject.cc to include
a header file that includes a header file generated at build time depending
on the PROTOCOL in use.  Since NULL ISA was not specifying any protocol,
this resulted in compilation problems.  Hence, the changeset.

build_opts/NULL

@@ -1,2 +1,3 @@
 TARGET_ISA = 'null'
 CPU_MODELS = ''
+PROTOCOL='MI_example'

src/mem/ruby/SConscript

@@ -59,9 +59,6 @@ CompoundFlag('Ruby', [ 'RubyQueue', 'RubyNetwork', 'RubyTester',
     'RubyMemory', 'RubyDma', 'RubyPort', 'RubySequencer', 'RubyCacheTrace',
     'RubyPrefetcher'])
 
-if env['TARGET_ISA'] == 'null':
-    Return()
-
 if env['PROTOCOL'] == 'None':
     Return()