Add Microsoft.DotNet.PlatformAbstractions package

Author: bording

LibGit2Sharp/LibGit2Sharp.csproj

@@ -33,6 +33,7 @@
 
   <ItemGroup>
     <PackageReference Include="LibGit2Sharp.NativeBinaries" Version="[1.0.215-pre20180422060246]" PrivateAssets="none" />
+    <PackageReference Include="Microsoft.DotNet.PlatformAbstractions" Version="2.0.4" />
     <PackageReference Include="Nerdbank.GitVersioning" Version="2.1.23" PrivateAssets="all" />
     <PackageReference Include="SourceLink.Create.GitHub" Version="2.8.0" PrivateAssets="all" />
     <DotNetCliToolReference Include="dotnet-sourcelink-git" Version="2.8.0" />