xen/pvcalls: Make pvcalls_back_global static

Fix sparse warning:

drivers/xen/pvcalls-back.c:30:3: warning:
symbol 'pvcalls_back_global' was not declared. Should it be static?

Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: YueHaibing <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Stefano Stabellini <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/pvcalls-back.c

···
 #define PVCALLS_VERSIONS "1"
 #define MAX_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
 
-struct pvcalls_back_global {
+static struct pvcalls_back_global {
 	struct list_head frontends;
 	struct semaphore frontends_lock;
 } pvcalls_back_global;