Fix a compilation error when using clang++

Clang gives this error:

  In file included from opencv_trail_effect.cpp:25:
  ./Trail.hpp:13:22: error: in-class initialization of non-static data member is a C++11 extension [-Werror,-Wc++11-extensions]
          cv::Mat *background = NULL;
                              ^
  1 error generated.
  <builtin>: recipe for target 'opencv_trail_effect.o' failed
  make: *** [opencv_trail_effect.o] Error 1

Fix it to make compilation succeed.