SDL_mixer: upgrade vendored wavpack to 5.9.0

From 56ee584f01717b46e94e4743459db29be8586a20 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Sun, 18 Jan 2026 17:30:40 +0300
Subject: [PATCH] upgrade vendored wavpack to 5.9.0

---
 .gitmodules      | 2 +-
 external/wavpack | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.gitmodules b/.gitmodules
index 7f2b11f84..2f2fd2ad1 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -33,7 +33,7 @@
 [submodule "external/wavpack"]
 	path = external/wavpack
 	url = https://github.com/libsdl-org/wavpack.git
-	branch = 5.8.1-SDL
+	branch = 5.9.0-SDL
 [submodule "external/libgme"]
 	path = external/libgme
 	url = https://github.com/libsdl-org/game-music-emu.git
diff --git a/external/wavpack b/external/wavpack
index 4382aacf5..c88aaf8fb 160000
--- a/external/wavpack
+++ b/external/wavpack
@@ -1 +1 @@
-Subproject commit 4382aacf5182e923de6e50ace807f8d122793e4e
+Subproject commit c88aaf8fb902c547a11053273df289d4b892335f