boxmodels: make them compile with -pedantic

tested on GCC 4.3 and GCC 4.6. Due to an incompatibility between those
two compilers (GCC 4.3 expects that there's no semicolon behind
static_assert(), GCC 4.6 requires one), it only compiles on GCC 4.6.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7288 2fb0f335-1f38-0410-981e-8018bf24f1b0