net: tipc: Add __printf() markup to fix -Wsuggest-attribute=format

net/tipc/netlink_compat.c: In function ‘tipc_tlv_sprintf’:
net/tipc/netlink_compat.c:137:2: warning: function ‘tipc_tlv_sprintf’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
  137 |  n = vscnprintf(buf, rem, fmt, args);

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>