Fix the folly.natvis references

Author: vmoroz

packages/playground/windows/playground-composition/Playground-Composition.vcxproj

@@ -144,9 +144,6 @@
     </Midl>
     <Midl Include="YogaXamlPanel.idl" />
   </ItemGroup>
-  <ItemGroup>
-    <Natvis Include="$(ReactNativeWindowsDir)Folly\Folly.natvis" />
-  </ItemGroup>
   <ItemGroup>
     <ResourceCompile Include="Playground-Composition.rc" />
   </ItemGroup>

vnext/Microsoft.ReactNative/Microsoft.ReactNative.vcxproj.filters

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <Natvis Include="$(ReactNativeWindowsDir)Folly\Folly.natvis" />
+    <Natvis Include="$(ExternalDir)folly\Folly.natvis" />
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="$(GeneratedFilesDir)module.g.cpp" />