Export CZString with JSON_API to fix Windows DLL linker errors

include/json/value.h

diff --git a/include/json/value.h b/include/json/value.h
index 8657e6c..a7a39a1 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -270,7 +270,7 @@
 private:
 #endif
 #ifndef JSONCPP_DOC_EXCLUDE_IMPLEMENTATION
-  class CZString {
+  class JSON_API CZString {
   public:
     enum DuplicationPolicy { noDuplication = 0, duplicate, duplicateOnCopy };
     CZString(ArrayIndex index);