fix include-order to fix build when shogun-headers present in system-dirs

As mentioned in https://github.com/shogun-toolbox/shogun/issues/1655: When building SHOGUN from sources and having an older / other version of it installed in the system's locations, e.g. /usr/local, the build fails, because the system-installed headers are used instead of the ones from sources.  This commit fixes that problem.