Add support for opencv-4

[zeroby0]

The current version of opencv is 4 and you get this build error with the current makefile:

g++ -std=c++11 -O2 -fstrict-aliasing -ffast-math `pkg-config --cflags opencv` ssimulacra.cpp `pkg-config --libs-only-L opencv` -lopencv_core -lopencv_highgui -lopencv_imgcodecs -lopencv_imgproc -o ssimulacra
Package opencv was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opencv' found
Package opencv was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opencv' found
ssimulacra.cpp:81:10: fatal error: opencv2/opencv.hpp: No such file or directory
   81 | #include <opencv2/opencv.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.
make: *** [Makefile:4: ssimulacra] Error 1

This PR makes compilation succeed with opencv4.

Credits:

• https://stackoverflow.com/a/58290585
• https://github.com/haeky/ssimulacra who also seems to have done the same thing independently @haeky