Don't erase background of wxSearchCtrl buttons twice

Set background style to indicate that wxSearchButton is painted entirely
by its wxEVT_PAINT handler.

This should eliminate potential flicker under MSW.

src/generic/srchctlg.cpp

@@ -162,7 +162,9 @@ public:
           m_search(search),
           m_eventType(eventType),
           m_bmp(bmp)
-    { }
+    {
+        SetBackgroundStyle(wxBG_STYLE_PAINT);
+    }
 
     void SetBitmapLabel(const wxBitmap& label)
     {