FT-93 Add function prototype (missed last commit unfortunately)

ft/node.h

@@ -146,9 +146,12 @@ public:
 
     int num_pivots() const;
 
-    // return: the sum of the keys sizes of each pivot
+    // return: the total size of this data structure
     size_t total_size() const;
 
+    // return: the sum of the keys sizes of each pivot (for serialization)
+    size_t serialized_size() const;
+
 private:
     inline size_t _align4(size_t x) const {
         return roundup_to_multiple(4, x);