Move server x509 certs into non-uwp compiler flags

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -90,11 +90,13 @@ namespace MQTTnet.Server
             return this;
         }
 
+#if !WINDOWS_UWP
         public MqttServerOptionsBuilder WithEncryptionCertificate(X509Certificate2 certificate)
         {
             _options.TlsEndpointOptions.X509Certificate = certificate;
             return this;
         }
+#endif
 
         public MqttServerOptionsBuilder WithEncryptionSslProtocol(SslProtocols value)
         {

Source/MQTTnet/Server/MqttServerTlsTcpEndpointOptions.cs

@@ -13,7 +13,9 @@ namespace MQTTnet.Server
 
         public byte[] Certificate { get; set; }
 
+#if !WINDOWS_UWP
         public X509Certificate2 X509Certificate { get; set; }
+#endif
 
         public IMqttServerCertificateCredentials CertificateCredentials { get; set; }