Update SDL2 version string in megasource build output

CMakeLists.txt

@@ -203,7 +203,7 @@ set(MEGA_LIBOGG_VER "1.3.2")
 set(MEGA_LIBVORBIS_VER "1.3.5")
 set(MEGA_LIBTHEORA_VER "1.1.1")
 set(MEGA_FREETYPE_VER "2.12.0")
-set(MEGA_SDL2_VER "2.23.1-85bbf8e")
+set(MEGA_SDL2_VER "2.28.1")
 set(MEGA_OPENAL_VER "1.22.0")
 set(MEGA_MODPLUG_VER "0.8.8.4")