[SPARK-20874][EXAMPLES] Add Structured Streaming Kafka Source to examples project

## What changes were proposed in this pull request?

Add Structured Streaming Kafka Source to the `examples` project so that people can run `bin/run-example StructuredKafkaWordCount ...`.

## How was this patch tested?

manually tested it.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #18101 from zsxwing/add-missing-example-dep.

examples/pom.xml

@@ -90,6 +90,12 @@
       <version>${project.version}</version>
       <scope>provided</scope>
     </dependency>
+    <dependency>
+      <groupId>org.apache.spark</groupId>
+      <artifactId>spark-sql-kafka-0-10_${scala.binary.version}</artifactId>
+      <version>${project.version}</version>
+      <scope>provided</scope>
+    </dependency>
     <dependency>
       <groupId>org.apache.commons</groupId>
       <artifactId>commons-math3</artifactId>