add missing includes for the build with use_libcxx_modules

This is to fix build error when we set use_libcxx_modules=true in
chromium build.

Bug: 40263312
Change-Id: I6c21e8efbc7fbcf88c02087e6a3c7ab8a6277370
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/127390
Auto-Submit: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

testing/helpers/event.cc

diff --git a/testing/helpers/event.cc b/testing/helpers/event.cc
index 9ed69a4..abcf9d6 100644
--- a/testing/helpers/event.cc
+++ b/testing/helpers/event.cc
@@ -4,6 +4,7 @@
 
 #include "testing/helpers/event.h"
 
+#include <stdint.h>
 #include <stdio.h>
 
 #include <string>