build: warnings from CUDA

[tomoaki0705]

System information (version)

• OpenCV => recent next branch ( 593a376 ) + recent contrib next branch ( d980cc3 )
• Operating System / Platform => Jetson TX2/TX1/Xavier/Nano
• Compiler => GCC 7.5.0/5.4.0

Detailed description

After #3370 , I see following warnings from cudalegacy module
I'll send a patch shortly

[ 68%] Building CXX object modules/cudalegacy/CMakeFiles/opencv_cudalegacy.dir/src/fgd.cpp.o
/opencv_contrib/modules/cudalegacy/src/fgd.cpp: In function ‘int {anonymous}::findForegroundRegions(cv::cuda::GpuMat&, cv::Mat&, std::vector<std::vector<cv::Point_<int> > >&, CvMemStorage*, const cv::cuda::FGDParams&)’:
/opencv_contrib/modules/cudalegacy/src/fgd.cpp:383:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < contours.size(); ++i)
                           ^
/opencv_contrib/modules/cudalegacy/src/fgd.cpp:388:57: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
             if (brect.area() < params.minArea || isHole && params.is_obj_without_holes)
                                                         ^
/opencv_contrib/modules/cudalegacy/src/fgd.cpp: At global scope:
/opencv_contrib/modules/cudalegacy/src/fgd.cpp:370:45: warning: unused parameter ‘storage’ [-Wunused-parameter]
                               CvMemStorage* storage, const FGDParams& params)
                                             ^
/opencv_contrib/modules/cudalegacy/src/fgd.cpp:348:10: warning: ‘void {anonymous}::seqToContours(CvSeq*, CvMemStorage*, cv::OutputArrayOfArrays)’ defined but not used [-Wunused-function]
     void seqToContours(CvSeq* _ccontours, CvMemStorage* storage, OutputArrayOfArrays _contours)
          ^

Steps to reproduce

cmake -DOPENCV_EXTRA_MODULES_PATH=/opencv_contrib/modules -DBUILD_EXAMPLES=true -DOPENCV_ENABLE_NONFREE=true -DWITH_CUDA=ON -DPYTHON2_EXECUTABLE= -DCMAKE_BUILD_TYPE=Release -DCPU_DISPATCH=NEON_DOTPROD ..
make all

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc