Clang 19 has become more strict about errors in member functions, which
results in errors building math/gfan:
src/gfanlib_matrix.h:123:18: error: no member named 'vectormultiply' in 'Matrix<typ>'
123 | ret[i]=a.vectormultiply(b.column(i));
| ~ ^
The `vectormultiply` method has been commented out by upstream for
unknown reasons, but the `operator*` method that references it is also
never used, so stub it out.
PR: 282851
Approved by: maintainer timeout (2 weeks)
MFH: 2024Q4
9020eeb540