message: fix complex conjugation
  there is no member function .conj()
diff: @@ -657,10 +657,9 @@ GKO_ATTRIBUTES GKO_INLINE constexpr xstd::enable_if_t<!is_complex<T>(), T> imag(
  * @return conjugate of the object (by default, the object itself)
  */
  template <typename T>
  -GKO_ATTRIBUTES GKO_INLINE xstd::enable_if_t<is_complex<T>(), T>
  -conj(const T &x)
  +GKO_ATTRIBUTES GKO_INLINE xstd::enable_if_t<is_complex<T>(), T> conj(const T &x)
  {
  - return x.conj();
  + return T{x.real(), -x.imag()};
  }
  template <typename T>
  @@ -733,8 +732,8 @@ GKO_INLINE GKO_ATTRIBUTES constexpr T get_superior_power(
  // it is put into the `gko` namespace, only enable `std::isfinite` when
  // compiling host code.
  template <typename T>
  -GKO_INLINE GKO_ATTRIBUTES xstd::enable_if_t<!is_complex_s<T>::value, bool>
  -isfinite(const T &value)
  +GKO_INLINE GKO_ATTRIBUTES xstd::enable_if_t<!is_complex<T>(), bool> isfinite(
  + const T &value)
  {
  return std::isfinite(value);
  }
  @@ -754,8 +753,8 @@ isfinite(const T &value)
  * they are neither +/- infinity nor NaN.
  */
  template <typename T>
  -GKO_INLINE GKO_ATTRIBUTES xstd::enable_if_t<is_complex_s<T>::value, bool>
  -isfinite(const T &value)
  +GKO_INLINE GKO_ATTRIBUTES xstd::enable_if_t<is_complex<T>(), bool> isfinite(
  + const T &value)
  {
  return isfinite(value.real()) && isfinite(value.imag());
  }
  