message: pkg-config: Drop broken include path
  The legacy `includedir` was removed in 4.0 (https://github.com/opencv/opencv/pull/12477) but it was not removed from the `.pc` file.
  This causes issues when trying to link against OpenCV using pkg-config in CMake.
diff: prefix=@prefix@
  exec_prefix=@exec_prefix@
  libdir=@libdir@
  -includedir_old=@includedir@/opencv
  -includedir_new=@includedir@
  +includedir=@includedir@
  Name: OpenCV
  Description: Open Source Computer Vision Library
  Version: @OPENCV_VERSION_PLAIN@
  Libs: @OPENCV_PC_LIBS@
  Libs.private: @OPENCV_PC_LIBS_PRIVATE@
  -Cflags: -I${includedir_old} -I${includedir_new}
  +Cflags: -I${includedir}
  