added netcore 2.1 build

Source/MQTTnet.AspnetCore/Client/Tcp/SocketSender.cs

@@ -5,6 +5,10 @@ using System.Diagnostics;
 using System.IO.Pipelines;
 using System.Net.Sockets;
 
+#if NETCOREAPP2_1
+using System.Runtime.InteropServices;
+#endif
+
 namespace MQTTnet.AspNetCore.Client.Tcp
 {
     public class SocketSender

Source/MQTTnet.AspnetCore/MQTTnet.AspNetCore.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netcoreapp2.1</TargetFrameworks>
     <Product />
     <Company />
     <Authors />

Tests/MQTTnet.Benchmarks/MQTTnet.Benchmarks.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <DebugType>Full</DebugType>
-    <TargetFramework Condition=" '$(OS)' == 'Windows_NT' ">net461</TargetFramework>
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net461;netcoreapp2.1</TargetFrameworks>
     <TargetFramework Condition=" '$(OS)' != 'Windows_NT' ">netcoreapp2.1</TargetFramework>
     <LangVersion>7.2</LangVersion>
   </PropertyGroup>