ccmake: fix curses dialog broken by refactoring

During refactoring in commit f6291eee25 (cmCursesMainForm: Modernize
with STL and ranged-for loops, 2019-02-10) a transformation of a loop
went wrong and editing the cmake cache with ccmake no longer works.
Make ccmake work again.

Fixes: #19008

Source/CursesDialog/cmCursesMainForm.cxx

@@ -669,7 +669,7 @@ void cmCursesMainForm::FillCacheManagerFromUI()
       this->CMakeInstance->GetState()->GetCacheEntryValue(cacheKey);
     if (existingValue) {
       std::string oldValue = existingValue;
-      std::string newValue = entry->GetValue();
+      std::string newValue = entry->Entry->GetValue();
       std::string fixedOldValue;
       std::string fixedNewValue;
       cmStateEnums::CacheEntryType t =