BUILD: quic: fix anonymous union for gcc-4.4

BUILD: quic: fix anonymous union for gcc-4.4

Building QUIC with gcc-4.4 on el6 shows this error:

src/xprt_quic.c: In function 'qc_release_lost_pkts':
src/xprt_quic.c:1905: error: unknown field 'loss' specified in initializer
compilation terminated due to -Wfatal-errors.
make: *** [src/xprt_quic.o] Error 1
make: *** Waiting for unfinished jobs....

Initializing an anonymous form of union like :
     struct quic_cc_event ev = {
          (...)
          .loss.time_sent = newest_lost->time_sent,
          (...)
     };

generates an error with gcc-4.4 but not when initializing the
fields outside of the declaration.