Update Directory.Build.props

Author: SimonCropp

src/Directory.Build.props

@@ -11,6 +11,7 @@
     <ResolveAssemblyReferencesSilent>true</ResolveAssemblyReferencesSilent>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+    <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
   </PropertyGroup>
   <ItemGroup>
     <Using Include="System.ReadOnlySpan&lt;System.Char&gt;" Alias="CharSpan" />