[bgpd] Make declarations match

These variables are const.
diff --git a/bgpd/bgp_route.c b/bgpd/bgp_route.c
index 65f54e7..01b3898 100644
--- a/bgpd/bgp_route.c
+++ b/bgpd/bgp_route.c
@@ -56,8 +56,8 @@
 #include "bgpd/bgp_vty.h"
 
 /* Extern from bgp_dump.c */
-extern char *bgp_origin_str[];
-extern char *bgp_origin_long_str[];
+extern const char *bgp_origin_str[];
+extern const char *bgp_origin_long_str[];
 
 static struct bgp_node *
 bgp_afi_node_get (struct bgp_table *table, afi_t afi, safi_t safi, struct prefix *p,