v8_initializer: Add cstring header

Using strlen? #include <cstring>!

Change-Id: Ie9f6ec80faba37708dc3c2b2302df1e4ab1a1b21
Reviewed-on: https://pdfium-review.googlesource.com/c/50450
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/v8_initializer.cpp b/testing/v8_initializer.cpp
index 49ef257..1083984 100644
--- a/testing/v8_initializer.cpp
+++ b/testing/v8_initializer.cpp
@@ -4,6 +4,8 @@
 
 #include "testing/v8_initializer.h"
 
+#include <cstring>
+
 #include "public/fpdfview.h"
 #include "testing/utils/file_util.h"
 #include "testing/utils/path_service.h"