bts_features: add feature BTS_FEAT_IPV6_NSVC

Previous both the IPA nanobts and the sysmobts has been using the IPv4 only OML
attribute NM_ATT_IPACC_NS_LINK_CFG.
A bts with BTS_FEAT_IPV6_NSVC supports IPv6 for NSVC (PCU<Gb>SGSN) using
the new OML attribute NM_ATT_OSMO_NS_LINK_CFG.

Change-Id: I9ef7949f66764b3c639e45eb440122e318da44a0
diff --git a/include/osmocom/gsm/bts_features.h b/include/osmocom/gsm/bts_features.h
index 22461a6..341ba40 100644
--- a/include/osmocom/gsm/bts_features.h
+++ b/include/osmocom/gsm/bts_features.h
@@ -25,6 +25,7 @@
 	BTS_FEAT_SPEECH_H_AMR,
 	BTS_FEAT_ETWS_PN,
 	BTS_FEAT_PAGING_COORDINATION,	/* BTS hands CS paging to PCU/PACCH */
+	BTS_FEAT_IPV6_NSVC,
 	_NUM_BTS_FEAT
 };