Build fails on latest GSL master commits

[leto]

I made a few trivial documentation changes and our build status on master went from green to red, so it seems that something in master changed to break our build:

https://travis-ci.org/leto/math--gsl/builds/465674081

Looks like the build fails here:

xs/SparseMatrix_wrap.2.5.c:2373:5: error: unknown type name ‘gsl_spmatrix_tree’
     gsl_spmatrix_tree *arg1 = (gsl_spmatrix_tree *) 0 ;