Fixed gpu_id for DLL/SO

AlexeyAB

2018-03-19 ced198e9390195875d743d77eadece99c7fd5b38

 src/demo.c

@@ -143,6 +143,7 @@
        printf("video file: %s\n", filename);
        cap = cvCaptureFromFile(filename);
    }else{
      printf("Webcam index: %d\n", cam_index);
        cap = cvCaptureFromCAM(cam_index);
    }