gsm: Add enum for Network Feature Support IE

Change-Id: Ie409fd163b612bc3e2d7b8bf22d720705f0f6af1
diff --git a/include/osmocom/gsm/protocol/gsm_04_08_gprs.h b/include/osmocom/gsm/protocol/gsm_04_08_gprs.h
index 86c5b01..dbac259 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08_gprs.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08_gprs.h
@@ -98,6 +98,7 @@
 	GSM48_IE_GMM_PDP_CTX_STATUS	= 0x32,	/* 10.5.7.1 */
 	GSM48_IE_GMM_PS_LCS_CAPA	= 0x33,	/* 10.5.5.22 */
 	GSM48_IE_GMM_GMM_MBMS_CTX_ST	= 0x35,	/* 10.5.7.6 */
+	GSM48_IE_GMM_NET_FEAT_SUPPORT	= 0xB0,	/* 10.5.5.23 */
 };
 
 enum gsm48_gprs_ie_sm {