isl_constraint_add_div: correctly set line to first and only constraint
When isl_constraint_add_div was introduced in
3fcebd4 (isl_constraint:
add limited support for adding divs), the equalities were still placed
before the inequalities in the constraint matrix.
This changed in
9eb1488 (isl_basic_map: put inequalities before equalities),
but the implementation of isl_constraint_add_div was not adapted accordingly.
This has probably gone unnoticed because isl_constraint_add_div has only
been used on equality constraints.