Add missing include to test_iterators.h

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@271074 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/test_iterators.h

@@ -12,6 +12,7 @@
 
 #include <iterator>
 #include <stdexcept>
+#include <cstddef>
 #include <cassert>
 
 #include "test_macros.h"