SDL_mixer: fix build in c90 mode after commit 680d8d0866816654

SDL Commits
1 
From d8b31d2cde96ea7ca5e25895bc1b5743c6041fdd Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Sat, 15 May 2021 00:00:20 +0300
Subject: [PATCH] fix build in c90 mode after commit 680d8d0866816654

---
 src/mixer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/mixer.c b/src/mixer.c
index b9bdab7..16e53dc 100644
--- a/src/mixer.c
+++ b/src/mixer.c
@@ -1083,11 +1083,12 @@ int Mix_Volume(int which, int volume)
 /* Set volume of a particular chunk */
 int Mix_VolumeChunk(Mix_Chunk *chunk, int volume)
 {
+    int prev_volume;
+
     if (chunk == NULL) {
         return(-1);
     }
-  
-    int prev_volume = chunk->volume;
+    prev_volume = chunk->volume;
     if (volume >= 0) {
         if (volume > MIX_MAX_VOLUME) {
             volume = MIX_MAX_VOLUME;