hv_debugfs: Make hv_debug_root static

Fix sparse warning:

drivers/hv/hv_debugfs.c:14:15: warning: symbol 'hv_debug_root' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20200403082845.22740-1-yuehaibing@huawei.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>