BUG/MEDIUM: stats: fix crash on 'dump stats-file'

BUG/MEDIUM: stats: fix crash on 'dump stats-file'

A crash occurs immediately when stats-file dump is requested by the
command-line. The issue is caused by the introduction of watchers when
iterating over the proxies list, which is now required with dynamic
backends.

  commit 20376c54e2166a0882b71b26326360786f79ebdb
  MINOR: stats: protect proxy iteration via watcher

The above patch initializes the new proxy watcher for stats dump and
HTML. However, this was forgotten for stats-file command context. Fix
this by adding the missing watcher_init() in cli_parse_show_stat().

No need to backport.