xen/spinlock: fix use of 0 as a null pointer constant

The constant 0 is used as a null pointer constant, in
violation of MISRA C:2012 Rule 11.9, in builds with
CONFIG_DEBUG_LOCK_PROFILE defined.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/xen/spinlock.h

@@ -94,7 +94,7 @@ struct lock_profile_qhead {
     int32_t                   idx;     /* index for printout */
 };
 
-#define _LOCK_PROFILE(name) { 0, #name, &name, 0, 0, 0, 0, 0 }
+#define _LOCK_PROFILE(name) { NULL, #name, &name, 0, 0, 0, 0, 0 }
 #define _LOCK_PROFILE_PTR(name)                                               \
     static struct lock_profile * const __lock_profile_##name                  \
     __used_section(".lockprofile.data") =                                     \