Add /* fallthru */ comment to stop gcc 7+ from warning

src/liblsquic/lsquic_chsk_stream.c

@@ -117,6 +117,7 @@ hsk_client_on_read (lsquic_stream_t *stream, struct lsquic_stream_ctx *sh)
         break;
     default:
         LSQ_WARN("lsquic_enc_session_handle_chlo_reply returned unknown value %d", s);
+        /* fallthru */
     case DATA_FORMAT_ERROR:
         LSQ_INFO("lsquic_enc_session_handle_chlo_reply returned an error");
         c_hsk->buf_in = NULL;

src/liblsquic/lsquic_frame_reader.c

@@ -422,6 +422,7 @@ prepare_for_payload (struct lsquic_frame_reader *fr)
             fr->fr_state.payload_length);
         fr->fr_callbacks->frc_on_error(fr->fr_cb_ctx, stream_id,
                                                 FR_ERR_HEADERS_TOO_LARGE);
+        /* fallthru */
     continue_skipping:
     default:
         fr->fr_state.by_type.skip_state.n_skipped = 0;