SDL_mixer: Update mixer.c to use SDL3 Atomic type

SDL Commits
1 
From f60e89edabc702ba9fbfd2a33a27ad378768b5ef Mon Sep 17 00:00:00 2001
From: Emily Banerjee <[EMAIL REDACTED]>
Date: Mon, 13 Mar 2023 20:42:03 +0000
Subject: [PATCH] Update mixer.c to use SDL3 Atomic type

---
 src/mixer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/mixer.c b/src/mixer.c
index 9e448ac2..5133f399 100644
--- a/src/mixer.c
+++ b/src/mixer.c
@@ -111,7 +111,7 @@ static void *music_data = NULL;
 static const char **chunk_decoders = NULL;
 static int num_decoders = 0;
 
-static SDL_atomic_t master_volume = { MIX_MAX_VOLUME };
+static SDL_AtomicInt master_volume = { MIX_MAX_VOLUME };
 
 int Mix_GetNumChunkDecoders(void)
 {