Introduce BTS_FEAT_ETWS_PN for communicating ETWS PN capability

As 3GPP doesn't specify how the BSC shall communicate ETWS Primary
Notifications over Abis/RSL, we have to use a vendor-specific RSL
message for this.  And in order to know if the peer supports this
feature, we introduces BTS_FEAT_ETWS_PN.

Change-Id: I89c24a81ada6627694a9632e87485a61cbd3e680
Related: OS#4046, OS#4047
diff --git a/include/osmocom/gsm/bts_features.h b/include/osmocom/gsm/bts_features.h
index e84f95b..7ead020 100644
--- a/include/osmocom/gsm/bts_features.h
+++ b/include/osmocom/gsm/bts_features.h
@@ -23,6 +23,7 @@
 	BTS_FEAT_SPEECH_F_EFR,
 	BTS_FEAT_SPEECH_F_AMR,
 	BTS_FEAT_SPEECH_H_AMR,
+	BTS_FEAT_ETWS_PN,
 	_NUM_BTS_FEAT
 };