import mtl::size for size of std::vector

src/amdis/linearalgebra/mtl/Constraints.hpp

@@ -130,6 +130,7 @@ namespace AMDiS
         ins[t.row][t.col] += t.value;
 
       if (setDiagonal) {
+        using mtl::size;
         for (std::size_t i = 0; i < size(left); ++i) {
           if (left[i]) {
             ins[i][i] = T(1);