MINOR: proto_htx: Add the function htx_return_srv_error()

Instead of using a function from the legacy HTTP, the HTX code now uses its own
one.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 102c5de..8683e26 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -89,6 +89,7 @@
 void htx_perform_server_redirect(struct stream *s, struct stream_interface *si);
 void htx_server_error(struct stream *s, struct stream_interface *si, int err, int finst, const struct buffer *msg);
 void htx_reply_and_close(struct stream *s, short status, struct buffer *msg);
+void htx_return_srv_error(struct stream *s, struct stream_interface *si);
 struct buffer *htx_error_message(struct stream *s);
 
 void debug_hdr(const char *dir, struct stream *s, const char *start, const char *end);