add extern "C" to iothub_client_properties.h (#2548)

iothub_client/inc/iothub_client_properties.h

@@ -66,6 +66,11 @@
 
 #include "iothub_client_core_common.h"
 
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
 /** @brief Current version of @p IOTHUB_CLIENT_PROPERTY_REPORTED structure.  */
 #define IOTHUB_CLIENT_PROPERTY_REPORTED_STRUCT_VERSION_1 1
 
@@ -289,4 +294,8 @@ MOCKABLE_FUNCTION(, void, IoTHubClient_Properties_DeserializerProperty_Destroy,
 */
 MOCKABLE_FUNCTION(, void, IoTHubClient_Properties_Deserializer_Destroy, IOTHUB_CLIENT_PROPERTIES_DESERIALIZER_HANDLE, propertiesDeserializerHandle);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* IOTHUB_CLIENT_PROPERTIES_H */