[SPARK-7677] [STREAMING] Add Kafka modules to the 2.11 build.

This is somewhat related to [SPARK-6154](https://issues.apache.org/jira/browse/SPARK-6154), though it only touches Kafka, not the jline dependency for thriftserver.

I tested this locally on 2.11 (./run-tests) and everything looked good (I had to disable mima, because `MimaBuild` harcodes 2.10 for the previous version -- that's another PR).

Author: Iulian Dragos <jaguarul@gmail.com>

Closes #6149 from dragos/issue/spark-2.11-kafka and squashes the following commits:

aa15d99 [Iulian Dragos] Add Kafka modules to the 2.11 build.

pom.xml

@@ -107,6 +107,8 @@
     <module>examples</module>
     <module>repl</module>
     <module>launcher</module>
+    <module>external/kafka</module>
+    <module>external/kafka-assembly</module>
   </modules>
 
   <properties>
@@ -1757,10 +1759,6 @@
         <jline.version>${scala.version}</jline.version>
         <jline.groupid>org.scala-lang</jline.groupid>
       </properties>
-      <modules>
-        <module>external/kafka</module>
-        <module>external/kafka-assembly</module>
-      </modules>
     </profile>
 
     <profile>