Fix WebSocket4Net extension nuspec.

Build/MQTTnet.Extensions.WebSocket4Net.nuspec

@@ -15,7 +15,22 @@
     <copyright>Copyright Christian Kratky 2016-2020</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 
     <dependencies>
-      <dependency id="MQTTnet" version="$nugetVersion" />
+    <group targetFramework="netstandard1.3">
+        <dependency id="MQTTnet" version="$nugetVersion" />
+        <dependency id="WebSocket4Net" version="0.15.2" />
+      </group>
+      <group targetFramework="netstandard2.0">
+        <dependency id="MQTTnet" version="$nugetVersion" />
+        <dependency id="WebSocket4Net" version="0.15.2" />
+      </group>
+      <group targetFramework="netstandard2.1">
+        <dependency id="MQTTnet" version="$nugetVersion" />
+        <dependency id="WebSocket4Net" version="0.15.2" />
+      </group>
+      <group targetFramework="uap10.0">
+        <dependency id="MQTTnet" version="$nugetVersion" />
+        <dependency id="WebSocket4Net" version="0.15.2" />
+      </group>
     </dependencies>
   </metadata>