Fix for encoding errors when building Java source

The generated OpenCV Java source can contain characters outside of ASCII on some systems – this patch allows the ant task to compile them.
(cherry picked from commit f3ee55e042)

modules/java/build.xml.in

@@ -8,8 +8,9 @@
     <!-- http://stackoverflow.com/questions/3584968/ant-how-to-compile-jar-that-includes-source-attachment -->
     <javac sourcepath="" srcdir="src" destdir="src" debug="on" includeantruntime="false" >
       <include name="**/*.java"/>
+      <compilerarg line="-encoding utf-8"/>
     </javac>
 
     <jar basedir="src" destfile="bin/@JAR_NAME@"/>
   </target>
-</project>
+</project>