SDL_mixer: Make sure MODPLUB_STATIC is defined when building static modplug library

SDL Commits
1 
From 32381dfc650573f59adb974582a24644496c3004 Mon Sep 17 00:00:00 2001
From: Anonymous Maarten <[EMAIL REDACTED]>
Date: Fri, 13 Oct 2023 14:03:46 +0200
Subject: [PATCH] Make sure MODPLUB_STATIC is defined when building static
 modplug library

---
 external/libmodplug | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/external/libmodplug b/external/libmodplug
index ac378ae7..1e032986 160000
--- a/external/libmodplug
+++ b/external/libmodplug
@@ -1 +1 @@
-Subproject commit ac378ae725e89195e6ffca6cfc68a64f16a1c650
+Subproject commit 1e03298678882ed986194be761fdf286db98e2fa