ghost: add / between an mntns root and a file path

(00.170031) Error (files-reg.c:515):  `- XFail [.criu.mntns.3xE0jR/15var/tmp/ibmiNsaA.cr.5.ghost] ghost: No such file or directory

Reported-by: Adrian Reber <adrian@lisas.de>
Cc: Adrian Reber <adrian@lisas.de>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>