Add DisableSourceLink Condition in Avalonia.csproj

packages/Avalonia/Avalonia.csproj

@@ -49,5 +49,5 @@
   <Import Project="..\..\build\SharedVersion.props" />
   <Import Project="..\..\build\NetFX.props" />
   <Import Project="..\..\build\CoreLibraries.props" />
-  <Import Project="..\..\build\SourceLink.props" />
+  <Import Project="..\..\build\SourceLink.props" Condition="'$(DisableSourceLink)' == ''" />
 </Project>