message: fix compile error of bitwise_not in gcc5.5
diff: @@ -626,7 +626,7 @@ GKO_DEFINE_SIMPLE_UNARY_OPERATION(unary_minus, -operand);
  GKO_DEFINE_SIMPLE_UNARY_OPERATION(logical_not, !operand);
  // unary bitwise
  -GKO_DEFINE_SIMPLE_UNARY_OPERATION(bitwise_not, ~operand);
  +GKO_DEFINE_SIMPLE_UNARY_OPERATION(bitwise_not, ~(operand));
  // common functions
  GKO_DEFINE_SIMPLE_UNARY_OPERATION(zero_operation, zero(operand));
  