Add missing c_str() and similar calls to samples in STL build.

Don't rely on implicit conversion of wxString to char*/wxChar*, it doesn't
exist in STL build. Add explicit c_str(), wc_str() or t_str() calls.

Closes #14420.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71820 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/regtest/regtest.cpp

@@ -1193,7 +1193,7 @@ const wxChar *RegTreeCtrl::TreeNode::FullName() const
         s_strName = Parent()->FullName();
         s_strName << wxT('\\') << m_strName;
 
-        return s_strName;
+        return s_strName.t_str();
     }
 }