[whitespace] Fix trailing whitespace.

No idea how the trailing whitespace managed to sneak into master.  But now my
stupid whitespace hook won't let me merge master into my branch to resolve
conflicts.

dune/istl/test/solvertest.cc

@@ -80,6 +80,6 @@ int main(int argc, char** argv)
 
   Dune::FCGSolver<BVector> solver4(fop, prec0, 1e-3,10,2,3);
   solver4.apply(x, b, res);
-  
+
   return 0;
 }