Add Checkstyle IllegalImport

Author: garydgregory

src/conf/checkstyle.xml

@@ -45,6 +45,7 @@ limitations under the License.
     <module name="ExplicitInitialization" />
     <module name="FallThrough" />
     <module name="FinalLocalVariable" />
+    <module name="IllegalImport" />
     <module name="ImportOrder">
       <property name="option" value="top"/>
       <property name="groups" value="java,javax,org"/>