apparmor: add rule for allowing remount of boot_id

apparmor: add rule for allowing remount of boot_id

commit 863845075d3f77d27c91bd9f47d2f8ddc4867bd5 in upstream only partially
fixes the apparmor deny for mounting boot_id (used for example for identifying
different boots with `journalctl`) inside the container.

Tested by editing the profile and replacing it disregarding the cache:
`apparmor_parser -W -T -r /etc/apparmor.d/usr.bin.lxc-start`

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>