Added missing includes for std::istream.

Thanks to Quentin Fiard for the report.

include/json/reader.h

@@ -11,6 +11,7 @@
 # include "value.h"
 #endif // if !defined(JSON_IS_AMALGAMATION)
 # include <deque>
+# include <iosfwd>
 # include <stack>
 # include <string>
 

src/lib_json/json_reader.cpp

@@ -13,6 +13,7 @@
 #include <cstdio>
 #include <cassert>
 #include <cstring>
+#include <istream>
 #include <stdexcept>
 
 #if defined(_MSC_VER)  &&  _MSC_VER >= 1400 // VC++ 8.0