[inspector] suppress build warnings on windows/clang

R=machenbach@chromium.org,thakis@chromium.org
BUG=v8:5258

Review-Url: https://codereview.chromium.org/2206983002
Cr-Commit-Position: refs/heads/master@{#38343}

gypfiles/standalone.gypi

@@ -453,6 +453,8 @@
       'clang_warning_flags': [
         # TODO(thakis): https://crbug.com/604888
         '-Wno-undefined-var-template',
+        # TODO(yangguo): issue 5258
+        '-Wno-nonportable-include-path',
       ],
       'conditions':[
         ['OS=="android"', {