add missing DNN header to opencv2/opencv.hpp

Author: Steven

include/opencv2/opencv.hpp

@@ -58,6 +58,9 @@
 #ifdef HAVE_OPENCV_FEATURES2D
 #include "opencv2/features2d.hpp"
 #endif
+#ifdef HAVE_OPENCV_DNN
+#include "opencv2/dnn.hpp"
+#endif
 #ifdef HAVE_OPENCV_FLANN
 #include "opencv2/flann.hpp"
 #endif