Fix typo causing build process to blat private.properties each time

Change-Id: I450d8f9135c95ecd6b7969ee3bfa3365ac261200
Reviewed-on: http://gerrit.dmdirc.com/2370
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

build-properties.xml

@@ -2,7 +2,7 @@
 <project name="DMDirc-properties" default="default" basedir=".">
     <description>Automatically initialises build properties</description>
 
-    <condition property="private.property.exists">
+    <condition property="private.properties.exists">
         <available file="nbproject/private/private.properties"/>
     </condition>