change [[nodiscard]] to FOLLY_NODISCARD for "fbcode/quic/codec/QuicPacketBuilder.h"
change [[nodiscard]] to FOLLY_NODISCARD for "fbcode/quic/codec/QuicPacketBuilder.h"

Summary:
ovrsource is not happy about [[nodiscard]] when sync: https://www.internalfb.com/intern/sandcastle/job/36028797332881199/
use FOLLY_NODISCARD instead

Reviewed By: sharmafb

Differential Revision: D23192099

fbshipit-source-id: 4a849c3281d9bc99ad328e5191e01a17a759b240

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst