add counter for connection attempts from BTS with unknown unit id

Add a global counter to the BSC which shows the number of failed
connections attempts due to a unit_id mismatch between the BSC
and the BTS.

Change-Id: I58866aff36a1c8463bf84b4392a5124ffeaa32ea
Related: OS#3245
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 6365938..6f854d8 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1128,6 +1128,7 @@
 	BSC_CTR_PAGING_ATTEMPTED,
 	BSC_CTR_PAGING_DETACHED,
 	BSC_CTR_PAGING_RESPONDED,
+	BSC_CTR_UNKNOWN_UNIT_ID,
 };
 
 static const struct rate_ctr_desc bsc_ctr_description[] = {
@@ -1140,6 +1141,8 @@
 	[BSC_CTR_PAGING_ATTEMPTED] = 		{"paging:attempted", "Paging attempts for a subscriber."},
 	[BSC_CTR_PAGING_DETACHED] = 		{"paging:detached", "Paging request send failures because no responsible BTS was found."},
 	[BSC_CTR_PAGING_RESPONDED] = 		{"paging:responded", "Paging attempts with successful response."},
+
+	[BSC_CTR_UNKNOWN_UNIT_ID] = 		{"abis:unknown_unit_id", "Connection attempts from unknown IPA CCM Unit ID."},
 };