xen-4.8: fix qemu-xen build error in memfd.c

Apply https://github.com/qemu/qemu/commit/75e5b70e6b5dcc4f2219992d7cffa462aa406af0
see also https://www.mail-archive.com/xen-devel@lists.xenproject.org/msg08648.html

cc @eelco @tstrobel @oxij