x86/hyperv: mark hyperv_init as __init function

This change was done upstream as part of 6b48cb5f8347 ("X86/Hyper-V:
Enlighten APIC access"), but that commit introduced a lot of new
functionality we don't want to backport.

This change eliminates a build warning.

Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>