Constify data structures

Constify more data structures. Make functions static.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
diff --git a/extensions/libxt_tcp.c b/extensions/libxt_tcp.c
index 6ab0e22..ed07667 100644
--- a/extensions/libxt_tcp.c
+++ b/extensions/libxt_tcp.c
@@ -66,7 +66,7 @@
 	unsigned int flag;
 };
 
-static struct tcp_flag_names tcp_flag_names[]
+static const struct tcp_flag_names tcp_flag_names[]
 = { { "FIN", 0x01 },
     { "SYN", 0x02 },
     { "RST", 0x04 },