Small Fix: included directory for OpenCV specific pcl header

I tested the kinfu app together with OpenCV and couldn't run it because of a directory failure in the following line (../gpu/kinfu/tools/kinfu_app.cpp): 
#include < pcl/gpu/utils/timers_opencv.hpp >
This line appears also in other files (e.g. ../gpu/kinfu/src/kinfu.cpp), so I tried a redundant inclusion of the directory path with CMake at the highest possible level.