Always fall back on wxPLATFORM_UNIX in wxEncodingConverter.

We could have ended up using (invalid) wxPLATFORM_CURRENT as an index into an
array if neither __WINDOWS__, nor __WXMAC__, nor __WXGTK__ nor __WXMOTIF__
were defined (which would be the case in e.g. wxX11 or wxQt ports). Fix this
by always falling back on wxPLATFORM_UNIX.

src/common/encconv.cpp

@@ -438,10 +438,10 @@ wxFontEncodingArray wxEncodingConverter::GetPlatformEquivalents(wxFontEncoding e
     {
 #if defined(__WINDOWS__)
         platform = wxPLATFORM_WINDOWS;
-#elif defined(__WXGTK__) || defined(__WXMOTIF__)
-        platform = wxPLATFORM_UNIX;
 #elif defined(__WXMAC__)
         platform = wxPLATFORM_MAC;
+#else
+        platform = wxPLATFORM_UNIX;
 #endif
     }