x86/xen: fix 'nopvspin' build error

Fix build error in x86/xen/ when PARAVIRT_SPINLOCKS is not enabled.

Fixes this build error:

../arch/x86/xen/smp_hvm.c: In function ‘xen_hvm_smp_init’:
../arch/x86/xen/smp_hvm.c:77:3: error: ‘nopvspin’ undeclared (first use in this function)
   nopvspin = true;

Fixes: 3d7746be ("x86/xen: Fix xen_hvm_smp_init() when vector callback not available")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210115191123.27572-1-rdunlap@infradead.orgSigned-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/smp_hvm.c

@@ -74,7 +74,9 @@ void __init xen_hvm_smp_init(void)
 	smp_ops.cpu_die = xen_hvm_cpu_die;
 
 	if (!xen_have_vector_callback) {
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
 		nopvspin = true;
+#endif
 		return;
 	}