net/tls: Fix unused function warning

If PROC_FS is not set, gcc warning this:

net/tls/tls_proc.c:23:12: warning:
 'tls_statistics_seq_show' defined but not used [-Wunused-function]

Use #ifdef to guard this.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>