MINOR: quic: adjust quic_frame flag manipulation

Replace a plain '=' operator by '|=' when setting quic_frame
QUIC_FL_TX_FRAME_LOST flag.

For the moment, this change has no impact as only two exclusive flags
are defined for quic_frame. On the edited code path we are certain that
QUIC_FL_TX_FRAME_ACKED is not set due to a previous if statement, so a
plain equal or a binary OR is strictly identical.

This change will be useful if new flags are defined for quic_frame in
the future. These new flags won't be resetted automatically thanks to
binary OR without explictly intended, which otherwise could easily lead
to new bugs.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index bcd99ae..e366e20 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -1783,7 +1783,7 @@
 				/* Mark this STREAM frame as lost. A look up their stream descriptor
 				 * will be performed to check the stream is not consumed or released.
 				 */
-				frm->flags = QUIC_FL_TX_FRAME_LOST;
+				frm->flags |= QUIC_FL_TX_FRAME_LOST;
 			}
 			LIST_APPEND(&tmp, &frm->list);
 			TRACE_DEVEL("frame requeued", QUIC_EV_CONN_PRSAFRM, qc, frm);