[gnc-commodity.cpp] gnc_quote_source_num_entries speedup

previously was allocating a vector copy, and measuring the distance
manually. return std::list size() which is maintained internally.

libgnucash/engine/gnc-commodity.cpp

@@ -301,8 +301,7 @@ get_quote_source_from_type (QuoteSourceType type)
  ********************************************************************/
 gint gnc_quote_source_num_entries(QuoteSourceType type)
 {
-    auto source{get_quote_source_from_type(type)};
-    return std::distance(source.begin(), source.end());
+    return get_quote_source_from_type(type).size();
 }