BUILD: http: rename error_message http_error_message to fix conflicts on RHEL

Duncan Hall reported a build issue on CentOS where error_message conflicts
with another system declaration when SSL is enabled. Rename the function.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 69d932c..e7ad8ff 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -108,7 +108,7 @@
 
 struct http_req_rule *parse_http_req_cond(const char **args, const char *file, int linenum, struct proxy *proxy);
 void free_http_req_rules(struct list *r);
-struct chunk *error_message(struct session *s, int msgnum);
+struct chunk *http_error_message(struct session *s, int msgnum);
 
 /* to be used when contents change in an HTTP message */
 #define http_msg_move_end(msg, bytes) do { \