fix build with libc++ 19
tools/eml2mbox.cpp:18:13: error: use of undeclared identifier 'time'

Index: tools/eml2mbox.cpp
--- tools/eml2mbox.cpp.orig
+++ tools/eml2mbox.cpp
@@ -6,6 +6,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <memory>
+#include <time.h>
 #include <libHX/io.h>
 #include <libHX/scope.hpp>
 #include <libHX/string.h>
