Renamed "struct _Mix_Music" to "struct Mix_Music"

(cherry picked from commit 2463fa9)

Author: slouken

include/SDL_mixer.h

@@ -266,7 +266,7 @@ typedef enum Mix_MusicType {
 /**
  * The internal format for a music chunk interpreted via codecs
  */
-typedef struct _Mix_Music Mix_Music;
+typedef struct Mix_Music Mix_Music;
 
 /**
  * Open the default audio device for playback.

src/music.c

@@ -60,7 +60,7 @@ static int music_volume = MIX_MAX_VOLUME;
 static Mix_Music * volatile music_playing = NULL;
 SDL_AudioSpec music_spec;
 
-struct _Mix_Music {
+struct Mix_Music {
     Mix_MusicInterface *interface;
     void *context;