contrib: Add a script to clean up in regular intervals

This script should be run from cron. It compresses and deletes older
files.
diff --git a/configure.ac b/configure.ac
index 9977ef6..2f4de9d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,4 +57,5 @@
     include/Makefile
     include/osmo-pcap/Makefile
     src/Makefile
+    contrib/Makefile
     Makefile)