Remove underscore for _SDL_iconv_t

And change the name, not to have the same for pointer and data

include/SDL3/SDL_stdinc.h

@@ -602,7 +602,7 @@ extern DECLSPEC float SDLCALL SDL_tanf(float x);
 #define SDL_ICONV_EINVAL    (size_t)-4
 
 /* SDL_iconv_* are now always real symbols/types, not macros or inlined. */
-typedef struct _SDL_iconv_t *SDL_iconv_t;
+typedef struct SDL_iconv_data_t *SDL_iconv_t;
 extern DECLSPEC SDL_iconv_t SDLCALL SDL_iconv_open(const char *tocode,
                                                    const char *fromcode);
 extern DECLSPEC int SDLCALL SDL_iconv_close(SDL_iconv_t cd);

src/stdlib/SDL_iconv.c

@@ -106,7 +106,7 @@ enum
 #define ENCODING_UCS4NATIVE  ENCODING_UCS4LE
 #endif
 
-struct _SDL_iconv_t
+struct SDL_iconv_data_t
 {
     int src_fmt;
     int dst_fmt;