nacl: Be more consistent about setting nacl_untrusted_build on targets that use untrusted.gypi

third_party/modp_b64/modp_b64_nacl.gyp

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'includes': [
     '../../native_client/build/untrusted.gypi',
   ],
   'targets': [
     {
       'target_name': 'modp_b64_nacl',
       'type': 'none',
       'variables': {
         'nlib_target': 'libmodp_b64_nacl.a',
         'build_glibc': 0,
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
+        'nacl_untrusted_build': 1,
       },
       'sources': [
         'modp_b64.cc',
         'modp_b64.h',
         'modp_b64_data.h',
       ],
     },
   ],
 }