https://github.com/Return-To-The-Roots/libsiedler2/commit/cb2994d5bb6407d4b12f681268e6b123d9b366f5.patch
https://bugs.gentoo.org/787299
--- a/external/libsiedler2/include/libsiedler2/XMIDI_TrackConverter.h
+++ b/external/libsiedler2/include/libsiedler2/XMIDI_TrackConverter.h
@@ -18,6 +18,7 @@
 #pragma once
 
 #include <array>
+#include <cstddef>
 #include <cstdint>
 #include <vector>