Add logback dependency in sentinel-demo-nacos-datasource to avoid log ClassNotFoundException thrown by Nacos client (#438)

sentinel-demo/sentinel-demo-nacos-datasource/pom.xml

@@ -29,6 +29,12 @@
             <groupId>com.alibaba</groupId>
             <artifactId>fastjson</artifactId>
         </dependency>
+
+        <dependency>
+            <groupId>ch.qos.logback</groupId>
+            <artifactId>logback-classic</artifactId>
+            <version>1.2.3</version>
+        </dependency>
     </dependencies>
 
     <build>