added opencv2/opencv.hpp header to VideoSource_Linux_OpenCV.cc

Hello,

When compiling PTAM, I get the following error:

>‘VideoCapture’ was not declared in this scope
for the VideoSource_Linux_OpenCV.cc file.

Including "opencv2/opencv.hpp" fixes this and PTAM compiles successfully. 
I guess this is needed in newer versions of OpenCV. (tested with OpenCV 3.0 beta)