No need to fwd declare these structs

The typedef below is sufficient.

include/ua_plugin_historydatabase.h

@@ -13,7 +13,6 @@
 
 _UA_BEGIN_DECLS
 
-struct UA_HistoryDatabase;
 typedef struct UA_HistoryDatabase UA_HistoryDatabase;
 
 struct UA_HistoryDatabase {

plugins/historydata/ua_plugin_history_data_gathering.h

@@ -28,7 +28,6 @@ typedef struct {
     void * userContext;
 } UA_HistorizingNodeIdSettings;
 
-struct UA_HistoryDataGathering;
 typedef struct UA_HistoryDataGathering UA_HistoryDataGathering;
 struct UA_HistoryDataGathering {
     void *context;