Remove QUIC_EXPORT_PRIVATE attribute from PathValidationReason enum class.

This is causing windows compile failure.
https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/1411764

PiperOrigin-RevId: 503021271

quiche/quic/core/quic_path_validator.h

diff --git a/quiche/quic/core/quic_path_validator.h b/quiche/quic/core/quic_path_validator.h
index 6074fd8..fe975b0 100644
--- a/quiche/quic/core/quic_path_validator.h
+++ b/quiche/quic/core/quic_path_validator.h
@@ -29,7 +29,7 @@
 
 class QuicConnection;
 
-enum class QUIC_EXPORT_PRIVATE PathValidationReason {
+enum class PathValidationReason {
   kReasonUnknown,
   kMultiPort,
   kReversePathValidation,