xen: x86: mark xen_find_pt_base as __init

gcc-4.6 causes a harmless link-time warning:

WARNING: vmlinux.o(.text.unlikely+0x48e): Section mismatch in reference from the function xen_find_pt_base() to the function .init.text:m2p()
The function xen_find_pt_base() references
the function __init m2p().
This is often because xen_find_pt_base lacks a __init
annotation or the annotation of m2p is wrong.

Newer compilers inline this function, so it never shows up, but marking
it __init is the right way to avoid the warning.

Fixes: 70e61199 ("xen: move p2m list if conflicting with e820 map")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>