Add .NET Standard 2.0 defines

Ix.NET/Source/Directory.build.targets

@@ -13,7 +13,7 @@
   <PropertyGroup Condition="'$(TargetFramework)' == 'net46' or '$(TargetFramework)' == 'net461'">
     <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR</DefineConstants>
   </PropertyGroup>
-  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1'">
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1' or '$(TargetFramework)' == 'netstandard2.0'">
     <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR;HAS_ASYNCENUMERABLE;HAS_ASYNCDISPOSABLE;BCL_HAS_CONFIGUREAWAIT;HAS_VALUETUPLE</DefineConstants>
   </PropertyGroup>