[SystemZ][z/OS] Add back include required for strnlen function

Author: abhina-sree

clang/lib/Lex/HeaderMap.cpp

@@ -18,6 +18,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/SystemZ/zOSSupport.h"
 #include <cstring>
 #include <memory>
 #include <optional>