poudriere jail update from source: syscall.mk does not exist

From: Graham Perrin <grahamperrin_at_freebsd.org>
Date: Sat, 29 Oct 2022 20:30:23 UTC
After updating to yesterday's aba921bd9e1869dae9ae4cc6e0c048f997401034, 
I aimed for a routine update of the jail that I used for poudriere.


poudriere jail -u -J 1 -j main
…
===> lib/libc (install)
make[5]: "/usr/src/lib/libc/sys/Makefile.inc" line 9: Cannot open 
/usr/src/sys/sys/syscall.mk
make[5]: Fatal errors encountered -- cannot continue
make[5]: stopped in /usr/src/lib/libc
*** Error code 1
…


How to proceed?

Thanks