Fix sanitize-rule-names test for scons.

test/sanitize-rule-names/sanitize-rule-names.gyp

 # Copyright (c) 2012 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'targets': [
     {
       'target_name': 's_test',
       'type': 'executable',
       'rules': [
         {
           # Make sure this rule name doesn't cause an invalid ninja file.
           'rule_name': 'assembler (gnu-compatible)',
-          'extension': 'X',
+          'extension': 'S',
           'outputs': ['outfile'],
           'msvs_cygwin_shell': 0,
           'msvs_quote_cmd': 0,
           'action': ['python', 'script.py', '<(RULE_INPUT_PATH)', 'outfile'],
         },
       ],
       'sources': [
-        'blah.X',
+        'blah.S',
         'hello.cc',
       ],
     },
   ],
 }