dnn: fix support drop for IE<2018R3

- fix build with IE 2018R3

modules/dnn/src/op_inf_engine.cpp

@@ -784,7 +784,11 @@ bool InfEngineBackendLayer::getMemoryShapes(const std::vector<MatShape> &inputs,
                                             std::vector<MatShape> &outputs,
                                             std::vector<MatShape> &internals) const
 {
+#if INF_ENGINE_VER_MAJOR_EQ(INF_ENGINE_RELEASE_2018R3)
+    InferenceEngine::ICNNNetwork::InputShapes inShapes = const_cast<InferenceEngine::CNNNetwork&>(t_net).getInputShapes();
+#else
     InferenceEngine::ICNNNetwork::InputShapes inShapes = t_net.getInputShapes();
+#endif
     InferenceEngine::ICNNNetwork::InputShapes::iterator itr;
     bool equal_flag = true;
     size_t i = 0;

modules/dnn/src/op_inf_engine.hpp

@@ -114,10 +114,8 @@ public:
 
     virtual size_t getBatchSize() const noexcept CV_OVERRIDE;
 
-#if INF_ENGINE_VER_MAJOR_GT(INF_ENGINE_RELEASE_2018R2)
+    virtual InferenceEngine::StatusCode AddExtension(const InferenceEngine::IShapeInferExtensionPtr& extension, InferenceEngine::ResponseDesc* resp) noexcept CV_OVERRIDE;
-    virtual InferenceEngine::StatusCode AddExtension(const InferenceEngine::IShapeInferExtensionPtr& extension, InferenceEngine::ResponseDesc* resp) noexcept;
+    virtual InferenceEngine::StatusCode reshape(const InputShapes& inputShapes, InferenceEngine::ResponseDesc* resp) noexcept CV_OVERRIDE;
-    virtual InferenceEngine::StatusCode reshape(const InputShapes& inputShapes, InferenceEngine::ResponseDesc* resp) noexcept;
-#endif
 
     void init(int targetId);