[libFuzzer] Add missing include on Darwin.

GitOrigin-RevId: 7ca2b9aac782c223a9b3d59af9572a574e3a21d8
diff --git a/FuzzerExtraCountersDarwin.cpp b/FuzzerExtraCountersDarwin.cpp
index 61f288c..2321ba8 100644
--- a/FuzzerExtraCountersDarwin.cpp
+++ b/FuzzerExtraCountersDarwin.cpp
@@ -9,6 +9,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "FuzzerPlatform.h"
+#include <cstdint>
 
 #if LIBFUZZER_APPLE