Fix for compilation error

Lorenzo Botti managed to trigger an error caused by clang 3.0
generating a destructor in fe_boundary.C where it only had a forward
declaration for some of the object's members.  Adding the full header
to there (and to fe_lagrange_vec.C, where the problem *might* also
crop up) fixes things for him, and these were the only two places I
could see where there might be an issue.