Fix headercheck for evalcflflux*.hh by including header.

Use calculated porosity (found by a unused warning)
(partly reviewed by bernd, thanks for helping with the fix)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10716 2fb0f335-1f38-0410-981e-8018bf24f1b0