x86: fix arch/x86/kernel/genx2apic_uv_x.c build warning when !CONFIG_HOTPLUG_CPU

Impact: cleanup, reduce size of the kernel image a bit

Fix:

  arch/x86/kernel/genx2apic_uv_x.c:403: warning: 'uv_heartbeat_disable' defined but not used

the function is only used when CONFIG_HOTPLUG_CPU is defined.

Signed-off-by: Richard A. Holden III <aciddeath@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>