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

GitOrigin-RevId: ee91f9b4a2b8cd6bb5d37ccd34ee7fb2c1401f99
diff --git a/lib/Lex/HeaderMap.cpp b/lib/Lex/HeaderMap.cpp
index a7b670f..588b32e 100644
--- a/lib/Lex/HeaderMap.cpp
+++ b/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>