lxc: fix perl-prototyep of mountpoint_hotplug

lxc: fix perl-prototyep of mountpoint_hotplug

The original commit b2de4c048ee50094593f4f8ffd18b6c346f7157a
copy-pasta'd the wrong prototype, missing two scalar $ arguments.

Until recently perl did not care as those things are only checked
_somewhat_ on "compile" (module load) times, and the one (single?)
call site in PVE::LXC::Config missed the `use PVE::LXC` statement,
and so the module-load did not see the wrong prototype and thus did
not cared, on runtime all is different anyway (what a mess).

The recent commit 11066f6bfdca5225a6f872d5664e6637ccb58dd6 added that
use statement and made package compilation implode, almost like
spooky actions in the time-space distance...

Fixes: b2de4c048ee50094593f4f8ffd18b6c346f7157a
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>