Shut up some clang warnings

src/libutil/args.hh

@@ -199,6 +199,8 @@ private:
 
 public:
 
+    virtual ~Command() { }
+
     std::string name() { return _name; }
 
     virtual void prepare() { };

src/nix/command.hh

@@ -49,6 +49,8 @@ struct App
 
 struct Installable
 {
+    virtual ~Installable() { }
+
     virtual std::string what() = 0;
 
     virtual Buildables toBuildables()