Use explicit opset of Unsqueeze from nGraph

The change is needed due to removing default opset namespace for Unsqueeze
in the scope of this refactoring activity: https://github.com/openvinotoolkit/openvino/pull/2767

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

modules/dnn/src/layers/prior_box_layer.cpp

@@ -607,7 +607,7 @@ public:
 
             auto priorBox = std::make_shared<ngraph::op::PriorBoxClustered>(slice_layer, slice_image, attrs);
             auto axis = std::make_shared<ngraph::op::Constant>(ngraph::element::i64, ngraph::Shape{1}, std::vector<int64_t>{0});
-            auto unsqueeze = std::make_shared<ngraph::op::Unsqueeze>(priorBox, axis);
+            auto unsqueeze = std::make_shared<ngraph::op::v0::Unsqueeze>(priorBox, axis);
             return Ptr<BackendNode>(new InfEngineNgraphNode(unsqueeze));
         }
         else
@@ -628,7 +628,7 @@ public:
 
             auto priorBox = std::make_shared<ngraph::op::PriorBox>(slice_layer, slice_image, attrs);
             auto axis = std::make_shared<ngraph::op::Constant>(ngraph::element::i64, ngraph::Shape{1}, std::vector<int64_t>{0});
-            auto unsqueeze = std::make_shared<ngraph::op::Unsqueeze>(priorBox, axis);
+            auto unsqueeze = std::make_shared<ngraph::op::v0::Unsqueeze>(priorBox, axis);
             return Ptr<BackendNode>(new InfEngineNgraphNode(unsqueeze));
         }
     }