proxmox-boot: re-add include of helper-functions

proxmox-boot: re-add include of helper-functions

this fixes ccfbe44f75316d41e271f68a725accbd90655456

in the patch I accidentally removed the include of the helper
functions - the code uses `warn` from there

reported in our enterprise support portal and in our community-forum:
https://forum.proxmox.com/threads/.114998

reproduced with a VM:
* legacy bios
* installed PVE 6.3 (so that p-b-t was not used for legacy systems)
* then upgraded to a bullseye snapshot of 01.09.2022
* then upgraded to the last bullseye point-release
- this patch fixes the issue there.

The issue should not occur on systems using p-b-t or not using zfs
for /.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>