lib/build.xml: Also set javac source to java6

"javac: target release 6 conflicts with default source release 1.7"
error will be emitted if a java7 compiler is used and target is set to 6
but not source.

lib/build.xml

@@ -20,6 +20,7 @@
 	srcdir="src/"
 	destdir="build/"
 	bootclasspath="${sdk-location}/platforms/${android.version}/android.jar"
+	source="6"
 	target="6"
 	/>
     <jar