https://bugs.gentoo.org/show_bug.cgi?id=363017

--- a/libicq2000/libicq2000/sigslot.h
+++ b/libicq2000/libicq2000/sigslot.h
@@ -82,6 +82,7 @@
 #ifndef SIGSLOT_H__
 #define SIGSLOT_H__
 
+#include <cstddef>
 #include <set>
 #include <list>