Keep nxml-prolog-end up to date (Bug#18871)

The change on 2016-01-16 "lisp/nxml: Use syntax-tables for comments"
removed the update of nxml-prolog-end, although it already failed to
work correctly after 2013-10-07 "* lisp/nxml/nxml-mode.el: Use
lexical-binding and syntax-propertize."
* lisp/nxml/nxml-rap.el: New function.
* lisp/nxml/nxml-mode.el (nxml-mode): Add it to
1 job for master in 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #2002


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:2s)