SDL_image: update vendored libpng to fix Android.mk builds (d8398)

From d839832d5350747735c326f07b51b6745e02ae8b Mon Sep 17 00:00:00 2001
From: Ozkan Sezer <[EMAIL REDACTED]>
Date: Tue, 14 Jan 2025 11:56:02 +0300
Subject: [PATCH] update vendored libpng to fix Android.mk builds

(cherry picked from commit 65f39b9a50887de1080d1b86d0bf5bb253ab61fb)
---
 external/libpng | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/external/libpng b/external/libpng
index 6ace07a4..68ff096f 160000
--- a/external/libpng
+++ b/external/libpng
@@ -1 +1 @@
-Subproject commit 6ace07a41a0a44b2d7e00a5c59770441b0646a6e
+Subproject commit 68ff096f781a935233cbe7972bd5fc45f586b948