Fix a compilation problem when OpenCV includes are not in standard include path

Before I made this change, compilation with make failed with the
bellow included error. I do not claim THIS is the way how it should
be fixed, but I want to tell you there is a problem and that this
fix works for me. Thanks for your consideration.

…
g++ -c main.cpp -O3 -Wall -Wextra -I /usr/local/cuda/include
In file included from main.cpp:28:
HW2.cpp:1:33: error: opencv2/core/core.hpp: No such file or directory
HW2.cpp:2:39: error: opencv2/highgui/highgui.hpp: No such file or directory
HW2.cpp:3:30: error: opencv2/opencv.hpp: No such file or directory
In file included from main.cpp:28:
HW2.cpp:9: error: ‘cv’ has not been declared
HW2.cpp:9: error: expected constructor, destructor, or type conversion before ‘imageInputRGBA’
HW2.cpp:10: error: ‘cv’ has not been declared
…