Fix compilation in wxUSE_STL=1 build after r71467.

Restore explicit conversion of wxString to char*, there is no implicit
conversion in wxUSE_STL build.

Use utf8_str() instead of c_str() removed by r71467 however as this is what we
need for wxGTK.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71501 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/taskbar.cpp

@@ -207,7 +207,7 @@ void wxTaskBarIcon::Private::SetIcon()
 #if wxUSE_TOOLTIPS
     const char *tip_text = NULL;
     if (!m_tipText.empty())
-        tip_text = m_tipText;
+        tip_text = m_tipText.utf8_str();
 
 #if GTK_CHECK_VERSION(2,10,0)
     if (m_statusIcon)