Fix native_proto_caster.h include order (pybind11 include must be first).

PiperOrigin-RevId: 466412805

Author: rwgk

pybind11_protobuf/native_proto_caster.h

@@ -1,11 +1,14 @@
 #ifndef PYBIND11_PROTOBUF_NATIVE_PROTO_CASTERS_H_
 #define PYBIND11_PROTOBUF_NATIVE_PROTO_CASTERS_H_
 
-#include <Python.h>
+// pybind11 includes have to be at the very top, even before Python.h
 #include <pybind11/cast.h>
 #include <pybind11/pybind11.h>
 #include <pybind11/pytypes.h>
 
+// IWYU
+#include <Python.h>
+
 #include <functional>
 #include <memory>
 #include <optional>