See https://github.com/cvut/kea-hook-userchk-ldap/issues/5

--- a/src/user_registry.h
+++ b/src/user_registry.h
@@ -16,6 +16,7 @@
 #include <user_data_source.h>
 
 #include <string>
+#include <ctime>
 
 namespace user_chk {