Circumvent dlib-opencv mismatch compilation error

[ptosi]

Avoid the following compilation failure arising from a mismatch between particular versions of dlib and OpenCV (see davisking/dlib#1949):

    In file included from /usr/include/dlib/opencv.h:10,
                     from src/head_pose_estimation.hpp:5,
                     from src/head_pose_estimation.cpp:11:
    /usr/include/dlib/opencv/cv_image.h: In instantiation of ‘dlib::cv_image<pixel_type>::cv_image(cv::Mat) [with pixel_type = dlib::bgr_pixel]’:
    src/head_pose_estimation.cpp:48:46:   required from here
    /usr/include/dlib/opencv/cv_image.h:37:29: error: conversion from ‘const cv::Mat’ to non-scalar type ‘IplImage’ requested
       37 |             IplImage temp = img;

[severin-lemaignan]

Merged, thanks.