EditUndoRemoveChars::GetStr must return a reference

...as ImpEditEngine::ImpRemoveChars (editeng/source/editeng/impedit2.cxx) calls

  pCurUndo->GetStr() += aStr;

Regression introduced with d5e11f5f "convert
editeng/source/editeng/editundo.hxx from String to OUString."

Change-Id: Iab3dd3b861bca715a90e89e3a63a6f298657367d

editeng/source/editeng/editundo.hxx

@@ -123,7 +123,7 @@ public:
     EditUndoRemoveChars(EditEngine* pEE, const EPaM& rEPaM, const OUString& rStr);
 
     const EPaM&     GetEPaM() { return aEPaM; }
-    OUString        GetStr() const { return aText; }
+    OUString &      GetStr() { return aText; }
 
     virtual void    Undo() override;
     virtual void    Redo() override;