Fix "include what you use" issue (#1625)

Fix IWYU for istreambuf_iterator.

Co-authored-by: Victor Hugo Vianna Silva <victorvianna@google.com>

Author: victorvianna

src/lib_json/json_reader.cpp

@@ -16,6 +16,7 @@
 #include <cstring>
 #include <iostream>
 #include <istream>
+#include <iterator>
 #include <limits>
 #include <memory>
 #include <set>