call size function with explicit mtl namespace, due to ambiguities

src/amdis/linearalgebra/mtl/Constraints.hpp

@@ -130,8 +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) {
+        for (std::size_t i = 0; i < mtl::size(left); ++i) {
           if (left[i]) {
             ins[i][i] = T(1);
             ins[i][at(left2right,i)] = T(-1);