Fix headercheck for evalcflflux*.hh

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

Dumux-Svn-Revison: 10716
Ported-By: Andreas Lauser <andreas.lauser@iws.uni-stuttgart.de>
fix 'make headercheck' issue in decoupled code

the problem only compiled if the PROBLEM macro was set to 2, which is
the only value used in the tests. for such reasons, preprocessor magic
should be avoided at (almost) any cost...