CLEANUP: server: add "const" to some message strings

"updater" is used in "read only" mode, so I add a const qualifier
to the variable declaration.
diff --git a/include/proto/server.h b/include/proto/server.h
index d75cc9f..99af0d4 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -39,7 +39,7 @@
 int srv_lastsession(const struct server *s);
 int srv_getinter(const struct check *check);
 int parse_server(const char *file, int linenum, char **args, struct proxy *curproxy, struct proxy *defproxy);
-int update_server_addr(struct server *s, void *ip, int ip_sin_family, char *updater);
+int update_server_addr(struct server *s, void *ip, int ip_sin_family, const char *updater);
 struct server *server_find_by_id(struct proxy *bk, int id);
 struct server *server_find_by_name(struct proxy *bk, const char *name);
 struct server *server_find_best_match(struct proxy *bk, char *name, int id, int *diff);