math/gfan: fix build with clang 19

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

math/gfan/files/patch-src_gfanlib__matrix.h

@@ -0,0 +1,18 @@
+--- src/gfanlib_matrix.h.orig	2017-06-20 14:47:37 UTC
++++ src/gfanlib_matrix.h
+@@ -115,6 +115,7 @@ template <class typ> class Matrix{ (public)
+         	  p[i][j]=s*(q[i][j]);
+       return p;
+     }
++#if 0
+   friend Matrix operator*(const Matrix& a, const Matrix& b)
+     {
+       assert(a.width==b.height);
+@@ -123,6 +124,7 @@ template <class typ> class Matrix{ (public)
+         ret[i]=a.vectormultiply(b.column(i));
+       return ret.transposed();
+     }
++#endif
+   /*  template<class T>
+     Matrix<T>(const Matrix<T>& c):v(c.size()){
+     for(int i=0;i<size();i++)v[i]=typ(c[i]);}