[1383] Correct nesting of #ifdef ... #endif

The only reason this wasn't picked up before was because this header
file was never included more than once in a .cc file.

src/lib/resolve/response_classifier.h

@@ -151,7 +151,7 @@ private:
         size_t size);
 };
 
-#endif // __RESPONSE_CLASSIFIER_H
-
 } // namespace resolve
 } // namespace isc
+
+#endif // __RESPONSE_CLASSIFIER_H