Fix build for LLVM's remove transitive includes (KhronosGroup#6802)

In the latest LLVM roll this file requires the iterator header for
std::back_inserter

I believe LLVM has recently removed transitive includes by default, see:


llvm/llvm-project@ce5b2e8

Author: gilhooleyd

source/val/validate_cfg.cpp

@@ -15,6 +15,7 @@
 #include <cassert>
 #include <functional>
 #include <iostream>
+#include <iterator>
 #include <map>
 #include <string>
 #include <tuple>