Add output dir for opale_to_hdoc converter

opale_to_hdoc/opale_to_hdoc.ant

@@ -22,7 +22,9 @@
 
 	<!-- Beginning -->
 	<target name="convert">
-		<delete dir="${OutputPath}"/>
+		<delete>
+          <fileset dir="${OutputPath}" excludes=".gitkeep"/>
+        </delete>
 		<mkdir dir="${OutputPath}"/>
 		<for param="filename">
             <path>