SDL_image: Set the branch for the SDL dependency

SDL Commits
1 
From 412bdc039c9de361584b68f77c59f9b0cc2843b3 Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Tue, 1 Aug 2023 08:16:17 -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 c65d8586..d96a0b71 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/jpeg"]
 	path = external/jpeg
 	url = https://github.com/libsdl-org/jpeg.git