Explicitly specify encoding="UTF-8" for replace filters in build.xml

Some localization files were improperly encoded (at least on Windows)

build.xml

@@ -25,7 +25,7 @@
     </copy>
 
     <!-- Replace tokens everything -->
-    <replace dir="${target.xpi.dir}">
+    <replace dir="${target.xpi.dir}" encoding="UTF-8">
       <replacefilter token="@ID@" value="${extension.id}" />
       <replacefilter token="@NAME@" value="${extension.name}" />
       <replacefilter token="@VERSION@" value="${extension.version}" />