Melchior FRANZ: fix SGPropertyNode::LAST_USED_ATTRIBUTE

fixes commit c782a32076,
(also see 38494a48d8 :) )

simgear/props/props.cxx

@@ -642,7 +642,7 @@ SGPropertyNode::trace_read () const
  * Last used attribute
  * Update as needed when enum Attribute is changed
  */
-const int SGPropertyNode::LAST_USED_ATTRIBUTE = USERARCHIVE;
+const int SGPropertyNode::LAST_USED_ATTRIBUTE = PRESERVE;
 
 /**
  * Default constructor: always creates a root node.

simgear/props/props.hxx

@@ -762,6 +762,8 @@ public:
     TRACE_WRITE = 32,
     USERARCHIVE = 64,
     PRESERVE = 128
+    // beware: if you add another attribute here,
+    // also update value of "LAST_USED_ATTRIBUTE".
   };