make GetUsageString() public, this is useful at least for the tests

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@53566 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/cmdline.h

@@ -430,5 +430,12 @@ public:
         @see SetLogo()
     */
     void Usage() const;
+
+    /**
+        Return the string containing the program usage description.
+
+        Call Usage() to directly show this string to the user.
+     */
+    wxString GetUsageString() const;
 };