enb: expose RRC inactivity timer

allow the user to specifiy the RRC inactiviy timer used in the eNB.
set the default value to 20s

Change-Id: I6a264b8dee893b883d40a48831cbb47eb15e1e7b
diff --git a/sysmocom/defaults.conf b/sysmocom/defaults.conf
index dd97456..2f2e317 100644
--- a/sysmocom/defaults.conf
+++ b/sysmocom/defaults.conf
@@ -112,6 +112,7 @@
   mnc: 70
   transmission_mode: 1
   num_cells: 1
+  inactivity_timer: 20000
   enable_measurements: false
   enable_dl_awgn: false
   dl_awgn_snr: 30