cpu/hotplug: Fix build error of using {add,remove}_cpu() with !CONFIG_SMP

cpu/hotplug: Fix build error of using {add,remove}_cpu() with !CONFIG_SMP

279dcf693ac7 ("virt: acrn: Introduce an interface for Service VM to
control vCPU") introduced {add,remove}_cpu() usage and it hit below
error with !CONFIG_SMP:

../drivers/virt/acrn/hsm.c: In function ‘remove_cpu_store’:
../drivers/virt/acrn/hsm.c:389:3: error: implicit declaration of function ‘remove_cpu’; [-Werror=implicit-function-declaration]
   remove_cpu(cpu);

../drivers/virt/acrn/hsm.c:402:2: error: implicit declaration of function ‘add_cpu’; [-Werror=implicit-function-declaration]
   add_cpu(cpu);

Add add_cpu() function prototypes with !CONFIG_SMP and remove_cpu() with
!CONFIG_HOTPLUG_CPU for such usage.

Fixes: 279dcf693ac7 ("virt: acrn: Introduce an interface for Service VM to control vCPU")
Cc: Stephen Rothwell <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Qais Yousef <[email protected]>
Reported-by: Randy Dunlap <[email protected]>
Reviewed-by: Qais Yousef <[email protected]>
Acked-by: Randy Dunlap <[email protected]> # build-tested
Signed-off-by: Shuo Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>