Use `target_include_directories` in Allen for includes in CMakeLists

Currently, every include file needed for a target defined in a CMakeList has to be specified in that CMakeList. This can be tedious for maintenance and is probably also the cause why compilation of an lb-dev Allen project does not work, when single sub-directories are checked out only. We should switch to the same method used by other LHCb projects, i.e. using target_include_directories.