Adds encoding attribute utf-8 arg to cooja compile command

tools/cooja/build.xml

@@ -71,7 +71,8 @@ The COOJA Simulator
   <target name="compile" depends="init">
     <mkdir dir="${build}"/>
     <javac srcdir="${java}" destdir="${build}" debug="on"
-           includeantruntime="false">
+           includeantruntime="false"
+           encoding="utf-8">
       <classpath>
         <pathelement path="."/>
         <pathelement location="lib/jdom.jar"/>