Remove erroneous include guard

[charjr]

The tail end of an include guard is provided
within the defs.h code blocks,
however #pragma once is used at the top instead.

godot-docs/tutorials/scripting/gdextension/gdextension_c_example.rst

Lines 156 to 172 in e4d463e

	.. code-block:: c
	#pragma once
	#include <stdbool.h>
	#include <stddef.h>
	#include <stdint.h>
	#if !defined(GDE_EXPORT)
	#if defined(_WIN32)
	#define GDE_EXPORT __declspec(dllexport)
	#elif defined(__GNUC__)
	#define GDE_EXPORT __attribute__((visibility("default")))
	#else
	#define GDE_EXPORT
	#endif
	#endif // ! GDE_EXPORT

godot-docs/tutorials/scripting/gdextension/gdextension_c_example.rst

Lines 486 to 503 in e4d463e

	.. code-block:: c
	...
	// The sizes can be obtained from the extension_api.json file.
	#ifdef BUILD_32
	#define STRING_NAME_SIZE 4
	#else
	#define STRING_NAME_SIZE 8
	#endif
	// Types.
	typedef struct
	{
	uint8_t data[STRING_NAME_SIZE];
	} StringName;
	#endif // DEFS_H

I am also failing to compile the example using scons:

/bin/ld: src/gdexample.os:/home/john/Godot/gd-extension-c-example/src/api.h:20: multiple definition of `constructors'; src/api.os:/home/john/Godot/gd-extension-c-example/src/api.h:20: first defined here

Which is either on my end (and I could use some help 🙏 ) or indicative of other problems within this page.

[dsnopek]

Thanks! Looks good to me :-)

[mhilbrunner]

Thank you! Merged.