SDL_mixer: Set the branch for the libgme dependency

SDL Commits
1 
From 941544e623bfc1e4f0e1a5c410eee72a29176110 Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Fri, 4 Aug 2023 06:30:10 +0300
Subject: [PATCH] Set the branch for the libgme dependency

---
 .gitmodules | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.gitmodules b/.gitmodules
index 99ed6ef8..741c4d92 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -37,3 +37,4 @@
 [submodule "external/libgme"]
 	path = external/libgme
 	url = https://github.com/libsdl-org/game-music-emu.git
+	branch = v0.6.3-SDL