nsAutoString i/f has changed, need to align code accordingly to avoid build

error.

gfx/src/xprint/nsFontMetricsXP.cpp

@@ -2675,7 +2675,7 @@ nsFontMetricsXP::FindGenericFont(nsFontSearch* aSearch)
   char name[128];
   if (mLangGroup) {
     nsAutoString pref = prefix;
-    pref.Append('.');
+    pref.AppendWithConversion('.');
     const PRUnichar* langGroup = nsnull;
     mLangGroup->GetUnicode(&langGroup);
     pref.Append(langGroup);