Add default ctor in ChangeList class

We need a default ctor in this class to allow writing - https://github.com/salesforce/p4-fusion/blob/e21ee286d973c534d506349d5dc7e3638e671b56/p4-fusion/main.cc#L297

Author: twarit-waikar

p4-fusion/commands/change_list.h

@@ -35,6 +35,7 @@ struct ChangeList
 	int filesDownloaded;
 	State state;
 
+	ChangeList() = default; // Defaulted so that vector<ChangeList>::resize() can be used.
 	ChangeList(const std::string& number, const std::string& description, const std::string& user, const int64_t& timestamp);
 
 	ChangeList(const ChangeList& other) = delete;