Bugfix/crosscompilation does not properly set include dir

[Kaisrlik]

After cross-compilation I observe the variable LIBTINS_INCLUDE_DIRS in libtinsConfig.cmake file contains wrong value. For the reason I slightly rework it and make the include directory part of the target

[Kaisrlik]

Hello Matias, I played with cmake and I cannot find a way how to fix LIBTINS_INCLUDE_DIRS to work in all cases. Based on the cmake configuration described in [1] current configuration is alright. In addition I've browsed different projects to see how are using it. All projects what I saw they were using it in way of "targets" - similar to the first commit what I sent. For the reason I've kept the setting and added target_include_directory. [1]: https://gitlab.kitware.com/cmake/community/wikis/doc/tutorials/How-to-create-a-ProjectConfig.cmake-file