tipc: fix non-const printf format arguments

Fix warnings from current gcc about using non-const strings as printf
args in TIPC. Compile tested only (not a TIPC user).

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/dbg.c b/net/tipc/dbg.c
index 29ecae8..1885a7e 100644
--- a/net/tipc/dbg.c
+++ b/net/tipc/dbg.c
@@ -258,7 +258,7 @@
 	}
 
 	if (pb->echo)
-		printk(print_string);
+		printk("%s", print_string);
 
 	spin_unlock_bh(&print_lock);
 }