Moved package reference System.Xml.XmlSerializer to netstandard1_3

Author: johelvisguzman

src/DotNetToolkit.Repository.Xml/DotNetToolkit.Repository.Xml.csproj

@@ -9,7 +9,7 @@
     <Description>A repository context provider for storing entities in a XML formatted file.</Description>
   </PropertyGroup>
 
-  <ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.3'">
     <PackageReference Include="System.Xml.XmlSerializer" Version="4.3.0" />
   </ItemGroup>