xen:pvhvm: Modpost section mismatch fix

Removing __init from check_platform_magic since it is called by
xen_unplug_emulated_devices in non-init contexts (It probably gets inlined
because of -finline-functions-called-once, removing __init is more to avoid
mismatch being reported).

Signed-off-by: Raghavendra D Prabhu <rprabhu@wnohang.net>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>