chantype_rsl2gsmtap(): Add entries for CBCH

This adds missing entries for CBCH in the conversion between RSL-style
channel numbers and GSMTAP channel types.

Without this change, you will see tons of messages like
virtphy[19865]: Wed Feb 26 16:16:28 2020 DVIRPHY <0002> gsmtapl1_if.c:267 MS 0000: Ignoring unknown channel type UNKNOWN (0)
if running virtphy of osmocom-bb with a BTS that broadcasts CBCH.

Change-Id: I19bbd2942adf441f58955ac896ef968bfd8aec5f
diff --git a/src/gsmtap_util.c b/src/gsmtap_util.c
index 2fb18a4..a4fe2d9 100644
--- a/src/gsmtap_util.c
+++ b/src/gsmtap_util.c
@@ -86,6 +86,12 @@
 	case RSL_CHAN_OSMO_PDCH:
 		ret = GSMTAP_CHANNEL_PDCH;
 		break;
+	case RSL_CHAN_OSMO_CBCH4:
+		ret = GSMTAP_CHANNEL_CBCH51;
+		break;
+	case RSL_CHAN_OSMO_CBCH8:
+		ret = GSMTAP_CHANNEL_CBCH52;
+		break;
 	}
 
 	if (link_id & 0x40)