dnn: update "guard" inline namespace

- differ from 3.4 branch

modules/dnn/src/init.cpp

@@ -46,7 +46,7 @@
 
 namespace cv {
 namespace dnn {
-CV__DNN_EXPERIMENTAL_NS_BEGIN
+CV__DNN_INLINE_NS_BEGIN
 
 static Mutex* __initialization_mutex = NULL;
 Mutex& getInitializationMutex()
@@ -132,5 +132,5 @@ void initializeLayerFactory()
     CV_DNN_REGISTER_LAYER_CLASS(LSTM,           LSTMLayer);
 }
 
-CV__DNN_EXPERIMENTAL_NS_END
+CV__DNN_INLINE_NS_END
 }} // namespace