x86/callthunks: Mark apply_relocation() as __init_or_module

Do it like the rest of the methods using it.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20231105213731.1878100-3-ubizjak@gmail.com