Fix perfboot.py to exit by Ctrl+C.

BUG: 22207911
Change-Id: I0cc41f834207efd2965483c8636bbc709e54358f
diff --git a/init/perfboot.py b/init/perfboot.py
index 2cd4699..c0693ab 100755
--- a/init/perfboot.py
+++ b/init/perfboot.py
@@ -139,6 +139,7 @@
         def notify_timeout():
             self._timedout = True
         self._timer = threading.Timer(timeout, notify_timeout)
+        self._timer.daemon = True
         self._timer.start()
 
     def is_timedout(self):