change MOZ_PHOENIX ifdef to MOZ_XUL_APP to help GTK2 builds of thunderbird

xpfe/bootstrap/nsNativeAppSupportGtk.cpp

@@ -64,7 +64,7 @@ nsSplashScreenGtk::~nsSplashScreenGtk()
 
 NS_IMETHODIMP nsSplashScreenGtk::Show()
 {
-#ifdef MOZ_PHOENIX
+#ifdef MOZ_XUL_APP
   if (!splash_xpm[0])
     return NS_OK;
 #endif