fix compile error : anonymous struct (#1177)

Author: dota17

Diff for: include/json/value.h

@@ -639,7 +639,7 @@ class JSON_API Value {
     ObjectValues* map_;
   } value_;
 
-  struct {
+  struct Bits {
     // Really a ValueType, but types should agree for bitfield packing.
     unsigned int value_type_ : 8;
     // Unless allocated_, string_ must be null-terminated.