compat/c23: drop nullptr_t

and do not use in rxtx.h compat

GCC 16 on MinGW with ucrt64 env defines nullptr but doesn't nullptr_t
in stddef.h. As per c23 standard, nullptr_t is defined as `typedef
typeof_unqual(nullptr)` but typeof is not supported in older compilers
(eg. since gcc < 13) so the compat may introduce problems on contrary
on eg gcc 11.

Author: MartinPulec

src/compat/c23.h

@@ -54,7 +54,6 @@
 #if __STDC_VERSION__ < 202311L
         #include <stddef.h>
         #define nullptr NULL
-        typedef void *nullptr_t;
 #endif
 
 // countof

src/rxtx.h

@@ -162,7 +162,7 @@ typedef struct rx_audio_frames *rxtx_recv_audio_frame_fn(void *state);
 typedef void  rxtx_send_shr_ptr_video_frame_fn(void *state,
                                                std::shared_ptr<video_frame>);
 #else
-typedef nullptr_t rxtx_send_shr_ptr_video_frame_fn;
+typedef void *rxtx_send_shr_ptr_video_frame_fn;
 #endif // defined __cplusplus
 /**
  * @param f  f->callbacks.dispose(f) must be called