ninja windows: make sure there's a trailing slash on $(VCInstallDir)

test/win/vs-macros/vcinstalldir.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': 'test_slash',
+      'type': 'none',
+      'msvs_cygwin_shell': '0',
+      'actions': [
+        {
+          'action_name': 'root',
+          'inputs': [],
+          'outputs': ['out'],
+          'action': ['python', 'test_exists.py', '$(VCInstallDir)', 'out']
+        },
+        {
+          'action_name': 'bin',
+          'inputs': [],
+          'outputs': ['out2'],
+          'action': ['python', 'test_exists.py', '$(VCInstallDir)bin', 'out2'],
+        },
+        {
+          'action_name': 'compiler',
+          'inputs': [],
+          'outputs': ['out3'],
+          'action': [
+              'python', 'test_exists.py', '$(VCInstallDir)bin\\cl.exe', 'out3'],
+        },
+      ],
+    },
+  ]
+}