Add new "chan:mode_modify_nack" counter to count RSL MODE MODIFY NACK

Those NACKs shouldn't happen in production, and if they do, you probably
want to have a more persistent figure than a line in the log file about
it.  Having counters allows the user to monitor this efficiently.

Change-Id: Ic82c6baaf4cb88d07bc5cdc200f8279cf130f396
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 7cd7539..cbb809b 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -205,6 +205,7 @@
 	BTS_CTR_CHAN_ACT_NACK,
 	BTS_CTR_RSL_UNKNOWN,
 	BTS_CTR_RSL_IPA_NACK,
+	BTS_CTR_MODE_MODIFY_NACK,
 };
 
 static const struct rate_ctr_desc bts_ctr_description[] = {
@@ -228,6 +229,7 @@
 	[BTS_CTR_CHAN_ACT_NACK] =		{"chan_act:nack", "Number of Channel Activations that the BTS NACKed"},
 	[BTS_CTR_RSL_UNKNOWN] =			{"rsl:unknown", "Number of unknown/unsupported RSL messages received from BTS"},
 	[BTS_CTR_RSL_IPA_NACK] =		{"rsl:ipa_nack", "Number of IPA (RTP/dyn-PDCH) related NACKs received from BTS"},
+	[BTS_CTR_MODE_MODIFY_NACK] =		{"chan:mode_modify_nack", "Number of Channel Mode Modify NACKs received from BTS"},
 };
 
 static const struct rate_ctr_group_desc bts_ctrg_desc = {