fix a couple of harmless warnings reported by g++ 4.3

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@55255 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/strconv.cpp

@@ -1959,7 +1959,7 @@ size_t wxMBConv_iconv::GetMBNulLen() const
         wxMutexLocker lock(self->m_iconvMutex);
 #endif
 
-        wchar_t *wnul = L"";
+        const wchar_t *wnul = L"";
         char buf[8]; // should be enough for NUL in any encoding
         size_t inLen = sizeof(wchar_t),
                outLen = WXSIZEOF(buf);