lib/x86/smp: Fix compiler warnings

Add missing include of desc.h for prototypes of setup_idt and
set_idt_entry and fix the declaration of ipi_data - the pointer is
volatile, not the pointed-to datum.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
[Correct the fix for ipi_data. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>