Fix signed/unsigned warning

[[Imported from SVN: r9476]]

dune/gfe/targetspacertrsolver.cc

@@ -57,7 +57,7 @@ void TargetSpaceRiemannianTRSolver<TargetSpace>::solve()
     // /////////////////////////////////////////////////////
     //   Trust-Region Solver
     // /////////////////////////////////////////////////////
-    for (int i=0; i<maxTrustRegionSteps_; i++) {
+    for (size_t i=0; i<maxTrustRegionSteps_; i++) {
 
         if (this->verbosity_ == Solver::FULL) {
             std::cout << "----------------------------------------------------" << std::endl;

dune/gfe/targetspacertrsolver.hh

@@ -61,7 +61,7 @@ protected:
     double initialTrustRegionRadius_;
 
     /** \brief Maximum number of trust-region steps */
-    int maxTrustRegionSteps_;
+    size_t maxTrustRegionSteps_;
 
     /** \brief Maximum number of multigrid iterations */
     int innerIterations_;