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 <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20200410115620.33024-1-yuehaibing@huawei.com
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>