x86, tsc: fix section mismatch warning

WARNING: vmlinux.o(.text+0x7950): Section mismatch in reference from the function native_calibrate_tsc() to the function .init.text:tsc_read_refs()
The function native_calibrate_tsc() references
the function __init tsc_read_refs().
This is often because native_calibrate_tsc lacks a __init
annotation or the annotation of tsc_read_refs is wrong.

tsc_read_refs is called from native_calibrate_tsc which is not __init
and native_calibrate_tsc cannot be marked __init

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>