explicit cast to get rid of a signed/unsigned warning

[[Imported from SVN: r8652]]

dune/solvers/solvers/quadraticipopt.hh

@@ -182,7 +182,7 @@ get_bounds_info(Ipopt::Index n, Ipopt::Number* x_l, Ipopt::Number* x_u,
 {
     // here, the n and m we gave IPOPT in get_nlp_info are passed back to us.
     // If desired, we could assert to make sure they are what we think they are.
-    assert(n == x_->dim());
+    assert(n == (Ipopt::Index)x_->dim());
     assert(m == 0);
 
     if (obstacles_) {