The function prototype is declared twice in include/haproxy/quic_conn.h.
Let's remove the second one.
struct quic_cstream *quic_cstream_new(struct quic_conn *qc);
void quic_cstream_free(struct quic_cstream *cs);
void quic_free_arngs(struct quic_conn *qc, struct quic_arngs *arngs);
-struct quic_cstream *quic_cstream_new(struct quic_conn *qc);
struct task *quic_conn_app_io_cb(struct task *t, void *context, unsigned int state);
void quic_conn_closed_err_count_inc(struct quic_conn *qc, struct quic_frame *frm);