Try changing your header to this, what you have should work:
#include "Runtime/UMG/Public/UMG.h"
#include "Runtime/UMG/Public/UMGStyle.h"
#include "Runtime/UMG/Public/Slate/SObjectWidget.h"
#include "Runtime/UMG/Public/IUMGModule.h"
#include "Runtime/UMG/Public/Blueprint/UserWidget.h"
UCLASS()
class OPENWORLDRPG_API UNewItemwidget : public UUserWidget
{
GENERATED_BODY()
public:
//your stuff ...