x86: make setup_xstate_init() __init

WARNING: vmlinux.o(.text+0x22453): Section mismatch in reference from the function setup_xstate_init() to the function .init.text:__alloc_bootmem()
The function setup_xstate_init() references the function __init __alloc_bootmem().
This is often because setup_xstate_init lacks a __init annotation or the annotation of __alloc_bootmem is wrong.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>