Pass correct licence path into plugins build.

Change-Id: I429f505fe870130067e0ba1be73973db335c9f35
Reviewed-on: http://gerrit.dmdirc.com/2731
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

build-plugins.xml

         <subant buildpath="modules/plugins" target="${plugins.build.target}">
             <property name="plugins.dmdirc.build.dir" value="../../${build.classes.dir}"/>
             <property name="clover.search.path" value="../../lib/test"/>
+            <property name="clover.license.path" location="etc/clover.license"/>
             <property name="clover.init.string" location=".clover/clover.db"/>
         </subant>
     </target>