Add missing QUIC_EXPORT_PRIVATE to fix QUIC merge
Add missing QUIC_EXPORT_PRIVATE to fix QUIC merge

gfe-relnote: n/a, chrome only change
PiperOrigin-RevId: 299077681
Change-Id: Ie3706e4bb5f0f25d2830d9397fb541efb6a1609b

google/quiche,google/quiche,google/quiche,google/quiche