build.xml: Add more directories to 'clean' target.

Change-Id: I05b8e425fb95a4e6e364c52b73d0857d60814ae9
diff --git a/build.xml b/build.xml
index e2859db..4d686aa 100644
--- a/build.xml
+++ b/build.xml
@@ -137,6 +137,8 @@
       <arg value="clean"/>
     </exec>
     <delete dir="${web-dir}/.parcel-cache"/>
+    <delete dir="${web-dir}/build"/>
+    <delete dir="${web-dir}/dist"/>
     <delete dir="${web-dir}/node_modules"/>
     <delete dir="${web-dir}/web_modules"/>
   </target>