[SPARK-13780][SQL] Add missing dependency to build.

This is needed to avoid odd compiler errors when building just the
sql package with maven, because of odd interactions between scalac
and shaded classes.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #11640 from vanzin/SPARK-13780.

sql/core/pom.xml

@@ -83,6 +83,10 @@
       <groupId>org.apache.parquet</groupId>
       <artifactId>parquet-hadoop</artifactId>
     </dependency>
+    <dependency>
+      <groupId>org.eclipse.jetty</groupId>
+      <artifactId>jetty-servlet</artifactId>
+    </dependency>
     <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>
       <artifactId>jackson-databind</artifactId>