MINOR: quic: Compilation fix for quic_rx_packet_refinc()

MINOR: quic: Compilation fix for quic_rx_packet_refinc()

This was reported by the CI wich clang as compilator.

    In file included from src/ssl_sock.c:80:
    include/haproxy/xprt_quic.h:1100:50: error: passing 'int *' to parameter of
    type 'unsigned int *' converts between pointers to integer types with
    different sign [-Werror,-Wpointer-sign]
        } while (refcnt && !HA_ATOMIC_CAS(&pkt->refcnt, &refcnt, refcnt - 1));
                                                        ^~~~~~~