explicitly call base class ctor to silent gcc warning (patch 1492701)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39375 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/strconv.cpp

@@ -1997,6 +1997,7 @@ public:
     }
 
     wxMBConv_win32(const wxMBConv_win32& conv)
+        : wxMBConv()
     {
         m_CodePage = conv.m_CodePage;
         m_minMBCharWidth = conv.m_minMBCharWidth;