Add missing QUIC_EXPORT_PRIVATE to test client ALPN
Add missing QUIC_EXPORT_PRIVATE to test client ALPN

gfe-relnote: n/a, Chromium-only change
PiperOrigin-RevId: 261369232
Change-Id: Ic277c6dc68f0d80549320d3abbca4f5cca050f2d

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