dnn(caffe): add DetectionOutputParameter.clip to .proto file

- allow to load opencv_face_detector with external unpatched protobuf

modules/dnn/src/caffe/opencv-caffe.proto

@@ -181,6 +181,8 @@ message DetectionOutputParameter {
   optional float confidence_threshold = 9;
   // If prior boxes are normalized to [0, 1] or not.
   optional bool normalized_bbox = 10 [default = true];
+  // OpenCV custom parameter
+  optional bool clip = 1000 [default = false];
 }
 
 message Datum {