Adds a "default" case within HeaderValidator::FinishHeaderBlock() to satisfy GCC.

PiperOrigin-RevId: 406424769

http2/adapter/header_validator.cc

diff --git a/http2/adapter/header_validator.cc b/http2/adapter/header_validator.cc
index fef2e2c..7cf9220 100644
--- a/http2/adapter/header_validator.cc
+++ b/http2/adapter/header_validator.cc
@@ -92,6 +92,8 @@
       return ValidateResponseHeaders(pseudo_headers_);
     case HeaderType::RESPONSE_TRAILER:
       return ValidateResponseTrailers(pseudo_headers_);
+    default:
+      return false;
   }
 }