ordered-set: add missing ordered_set_size()

src/basic/ordered-set.h

@@ -34,6 +34,10 @@ static inline int ordered_set_put(OrderedSet *s, void *p) {
         return ordered_hashmap_put((OrderedHashmap*) s, p, p);
 }
 
+static inline unsigned ordered_set_size(OrderedSet *s) {
+        return ordered_hashmap_size((OrderedHashmap*) s);
+}
+
 static inline bool ordered_set_isempty(OrderedSet *s) {
         return ordered_hashmap_isempty((OrderedHashmap*) s);
 }