update aux mkfile for xlisten

Reference: /n/atom/patch/applied2013/xlistenmk
Date: Thu Jul  4 00:50:04 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/aux/mkfile	Thu Jul  4 00:49:58 2013
+++ /sys/src/cmd/aux/mkfile	Thu Jul  4 00:49:58 2013
@@ -38,6 +38,7 @@
 	trampoline\
 	usage\
 	write\
+	xlisten\
 	zerotrunc\
 
 BIN=/$objtype/bin/aux