Fix include dir in Cmake

[sin3point14]

I added RTree as a submodule in my project and include it by using add_subdirectory in my cmake. However that gave warnings like:

cc1plus: warning: /home/sin3point14/my_projs/openargus/vendor/RTree/RTree.h: not a directory

and compilation fails as compiler couldn't find the header file. This fixes it.