fix strnlen() usage in Context.cpp available on z/OS

Author: zibi2

clang/lib/AST/ByteCode/Context.cpp

@@ -18,6 +18,7 @@
 #include "clang/AST/ASTLambda.h"
 #include "clang/AST/Expr.h"
 #include "clang/Basic/TargetInfo.h"
+#include "llvm/Support/SystemZ/zOSSupport.h"
 
 using namespace clang;
 using namespace clang::interp;