x86/apic/vsmp: Make is_vsmp_box() static

x86/apic/vsmp: Make is_vsmp_box() static

Since checkin

411cf9ee2946 x86, vsmp: Remove is_vsmp_box() from apic_is_clustered_box()

... is_vsmp_box() is only used in vsmp_64.c and does not have any
header file declaring it, so make it explicitly static.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Shai Fultheim <shai@scalemp.com>
Cc: Oren Twaig <oren@scalemp.com>
Link: http://lkml.kernel.org/r/1404036068-11674-1-git-send-email-oren@scalemp.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>