Use namespace std for int64_t and uint64_t

According to the c++11 standard, the declarations are within namespace
scope of the namespace `std'. Add `std::' to avoid unnecessary
requirement of implementations.

src/json.hpp

@@ -183,8 +183,8 @@ template <
     template<typename U, typename... Args> class ArrayType = std::vector,
     class StringType = std::string,
     class BooleanType = bool,
-    class NumberIntegerType = int64_t,
-    class NumberUnsignedType = uint64_t,
+    class NumberIntegerType = std::int64_t,
+    class NumberUnsignedType = std::uint64_t,
     class NumberFloatType = double,
     template<typename U> class AllocatorType = std::allocator
     >

src/json.hpp.re2c

@@ -183,8 +183,8 @@ template <
     template<typename U, typename... Args> class ArrayType = std::vector,
     class StringType = std::string,
     class BooleanType = bool,
-    class NumberIntegerType = int64_t,
-    class NumberUnsignedType = uint64_t,
+    class NumberIntegerType = std::int64_t,
+    class NumberUnsignedType = std::uint64_t,
     class NumberFloatType = double,
     template<typename U> class AllocatorType = std::allocator
     >