Multiple const members of struct breaks PYBIND11_NUMPY_DTYPE

Issue description

Attempting to PYBIND11_NUMPY_DTYPE on a struct containing more than one const double gives a ImportError: NumPy: invalid buffer descriptor! error on import. A class containing a single const double runs correctly. If you change the struct to contain two const double's or a const double and a regular double, it fails on import.

Reproducible example code

#include <pybind11/pybind11.h>
#include <pybind11/numpy.h>

namespace py = pybind11;

// a struct with one enum and a double
struct StructType1 {

  const double x;

  StructType1(const double x_) : x(x_) {};
  
};


// a struct with one enum and two doubles
struct StructType2 {

  const double x;
  const double y;

  StructType2(const double x_,
              const double y_) : x(x_), y(y_) {};


};


PYBIND11_MODULE(mystruct, m) {

  // wrap our class with one double
  py::class_<StructType1>(m, "StructType1")
    .def(py::init<const double>());

  // and wrap our class with two doubles
  py::class_<StructType2>(m, "StructType2")
    .def(py::init<const double, const double>());

  // this line is fine
  PYBIND11_NUMPY_DTYPE(StructType1, x);

  // including the below line gives a
  // ImportError: NumPy: invalid buffer descriptor!
  // PYBIND11_NUMPY_DTYPE(StructType2, x, y);

}