fixed detection_based_tracker on visual studio 2013 and later

modules/objectdetect/src/detection_based_tracker.cpp: made unique_lock<mutex> local to each function
samples/cpp/dbt_face_detection.cpp: fixed warnings on loop in Visual Studio

modules/objdetect/include/opencv2/objdetect/detection_based_tracker.hpp

@@ -59,7 +59,7 @@ namespace cv
 class CV_EXPORTS DetectionBasedTracker
 {
     public:
-        struct Parameters
+        struct CV_EXPORTS Parameters
         {
             int maxTrackLifetime;
             int minDetectionPeriod; //the minimal time between run of the big object detector (on the whole frame) in ms (1000 mean 1 sec), default=0