Return a const reference to self in nocopy.

While not technically necessary, it reduces warnings in cppcheck.

nocopy.hpp

@@ -28,7 +28,7 @@ class nocopy {
 
     private:
         nocopy            (const nocopy &) {                             ; }
-        nocopy& operator =(const nocopy &) { unreachable (); }
+        nocopy& operator =(const nocopy &) { unreachable (); return *this; }
 };
 
 #endif