The opencv element includes were full of duplicates and uneeded headers.
For example a few elements that stopped using gstcvopencvutils still
included that header file.
Change the gstretinex.c file to cpp and add it into Makefile.
It is necessary to migrate the retinex element to C++,
because new Opencv API leaves obsolete functions like cvSmooth.
This element uses this function.
You can see in this link:
http://docs.opencv.org/modules/imgproc/doc/filtering.html?
highlight=cvsmooth#void cvSmooth(const CvArr* src, CvArr* dst,
int smoothtype, int size1, int size2, double sigma1, double sigma2)
https://bugzilla.gnome.org/show_bug.cgi?id=754148
