Add missing header include that makes Chromium build fail.

PiperOrigin-RevId: 401578191

http2/adapter/header_validator_test.cc

diff --git a/http2/adapter/header_validator_test.cc b/http2/adapter/header_validator_test.cc
index a2a4cd5..01460c9 100644
--- a/http2/adapter/header_validator_test.cc
+++ b/http2/adapter/header_validator_test.cc
@@ -1,5 +1,6 @@
 #include "http2/adapter/header_validator.h"
 
+#include "absl/strings/str_cat.h"
 #include "common/platform/api/quiche_test.h"
 
 namespace http2 {