remoting: Suppress a clang warning that fires in atl headers.

Suppresses

In file included from ..\..\remoting/host/verify_config_window_win.h:10:
..\..\third_party\wtl\include\atlapp.h(1683,17) :  warning(clang): using namespace directive in global context in header [-Wheader-hygiene]
using namespace WTL;
                ^

BUG=5027,82385
TBR=jamiewalch@chromium.org

Review URL: https://codereview.chromium.org/526863002

Cr-Commit-Position: refs/heads/master@{#292821}

remoting/remoting_host_win.gypi

@@ -250,6 +250,16 @@
                 '/EXPORT:DllUnregisterServer=PsDllUnregisterServer,PRIVATE',
               ],
             },
+            'conditions': [
+              ['clang==1', {
+                # atlbase.h contains a global "using namespace WTL;".
+                # TODO: Remove once remoting/host/verify_config_window_win.h no
+                # longer depends on atlbase.h, http://crbug.com/5027
+                'VCCLCompilerTool': {
+                  'AdditionalOptions': ['-Wno-header-hygiene'],
+                },
+              }],
+            ],
           },
         },  # end of target 'remoting_core'
         {