[oncall] Fix BUILD error for "unused `trust_anchor_id`" when `#if/#endif` block is not entered.

Part of quic envoy merge duties this week.

Protected by FLAGS_gfe2_reloadable_flag_enable_tls_trust_anchor_ids.

PiperOrigin-RevId: 758414617

quiche/quic/core/crypto/tls_server_connection.cc

diff --git a/quiche/quic/core/crypto/tls_server_connection.cc b/quiche/quic/core/crypto/tls_server_connection.cc
index 39e2f4a..2b7ddc7 100644
--- a/quiche/quic/core/crypto/tls_server_connection.cc
+++ b/quiche/quic/core/crypto/tls_server_connection.cc
@@ -95,6 +95,8 @@
           trust_anchor_id.size());
       SSL_CREDENTIAL_set_must_match_issuer(credential, 1);
     }
+#else
+    (void)trust_anchor_id;  // Suppress unused parameter error.
 #endif
     SSL_add1_credential(ssl(), credential);
     SSL_CREDENTIAL_free(credential);