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 <[e-mail]>
Signed-off-by: YueHaibing <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tls/tls_proc.c

···
 #include <net/snmp.h>
 #include <net/tls.h>
 
+#ifdef CONFIG_PROC_FS
 static const struct snmp_mib tls_mib_list[] = {
 	SNMP_MIB_ITEM("TlsCurrTxSw", LINUX_MIB_TLSCURRTXSW),
 	SNMP_MIB_ITEM("TlsCurrRxSw", LINUX_MIB_TLSCURRRXSW),
···
 
 	return 0;
 }
+#endif
 
 int __net_init tls_proc_init(struct net *net)
 {