Fix follow-up warning on comparing signed and unsigned values

dune/dorie/common/grid_mapper.hh

@@ -904,7 +904,7 @@ private:
     template<typename Vec1, typename Vec2>
     void copy_if_not_ignored (const Vec1& vec1,
                               Vec2& vec2,
-                              const int ignored) const
+                              const size_t ignored) const
     {
         auto it = vec2.begin();
         for (size_t i = 0; i < vec1.size(); ++i)