nat: Count the number of dropped SCCP and MGCP calls as well.

Count the number of dropped calls and lost SCCP connections
due the loss of a connection to the BSC...
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index d211853..ca235bf 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -43,6 +43,8 @@
 	[BCFG_CTR_SCCP_CONN]     = { "sccp.conn", "SCCP Connections" },
 	[BCFG_CTR_SCCP_CALLS]    = { "sccp.calls", "SCCP Assignment Commands"},
 	[BCFG_CTR_NET_RECONN]    = { "net.reconnects", "Network reconnects"},
+	[BCFG_CTR_DROPPED_SCCP]  = { "dropped.sccp", "Dropped SCCP connections."},
+	[BCFG_CTR_DROPPED_CALLS] = { "dropped.calls", "Dropped active calls."},
 };
 
 static const struct rate_ctr_group_desc bsc_cfg_ctrg_desc = {