powerpc/xics: Add __init to marker icp_native_init()

This should fix the following warning:

 LD      arch/powerpc/sysdev/xics/built-in.o
WARNING: arch/powerpc/sysdev/xics/built-in.o(.text+0x1310): Section mismatch in
reference from the function .icp_native_init() to the function
.init.text:.icp_native_init_one_node()
The function .icp_native_init() references
the function __init .icp_native_init_one_node().
This is often because .icp_native_init lacks a __init
annotation or the annotation of .icp_native_init_one_node is wrong.

icp_native_init() is only referenced in `arch/powerpc/sysdev/xics/xics-common.c'
by xics_init() which is itself marked with __init.

= not built-tested =

Reported-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>