SDL_mixer: Set the branch for the SDL dependency

From d1a82962c6fc6e06c4a75bb0982fa9a99c53480d Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Tue, 1 Aug 2023 08:17:13 -0700
Subject: [PATCH] Set the branch for the SDL dependency

This fixes download.sh
---
 .gitmodules | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.gitmodules b/.gitmodules
index cff9121d..fc3b1474 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,6 +1,7 @@
 [submodule "external/SDL"]
 	path = external/SDL
 	url = https://github.com/libsdl-org/SDL.git
+	branch = main
 [submodule "external/flac"]
 	path = external/flac
 	url = https://github.com/libsdl-org/flac.git