Add ThirdPartyNotice file to packages - Fixes #4158

Author: michael-hawker

build/Windows.Toolkit.Common.targets

@@ -15,6 +15,7 @@
   <ItemGroup Condition="$(IsPackable)">
     <None Include="$(BuildToolsDirectory)nuget.png" Pack="true" PackagePath="\Icon.png" />
     <None Include="$(RepositoryDirectory)License.md" Pack="true" PackagePath="\" />
+    <None Include="$(RepositoryDirectory)ThirdPartyNotices.txt" Pack="true" PackagePath="\" />
   </ItemGroup>
 
   <ItemGroup>