workaround, make the opaque struct MatrixElement partially visible
this struct is private to the `sparse' world,
and lives in "maths/sparse/spdefs.h"
which can't be included here, because
it is incompatible to the rest of ngspice.
this is the same problem as in
ciderlib/oned/oneprint.c
ciderlib/twod/twoprint.c
checked for object file invariance on linux with
./configure --enable-debug=no --enable-maintainer-mode --with-readline=yes --enable-cider --enable-ndev --enable-pss --enable-xspice