ifdef and comment out libjpeg

Chances are there are not many themes in widespread use that are
using jpeg images, but this will be easy to revert back or even
just temporarily modify if we need jpeg support in a pinch.

Change-Id: If17b05ce3ebccce724fd74b26ed8ea91b287c3cf
diff --git a/gui/theme/portrait_mdpi/ui.xml b/gui/theme/portrait_mdpi/ui.xml
index 4d002ff..5ac6b95 100644
--- a/gui/theme/portrait_mdpi/ui.xml
+++ b/gui/theme/portrait_mdpi/ui.xml
@@ -5,7 +5,7 @@
 		<author>TeamWin</author>
 		<title>Backup Naowz</title>
 		<description>Default basic theme</description>
-		<preview>preview.jpg</preview>
+		<preview>preview.png</preview>
 		<themeversion>1</themeversion>
 	</details>