Add function to make Uplink Measurement

It assembles RSL Uplink Measurement IE according to 3GPP TS 08.58
ยง9.3.25. The function is based on lchan_build_rsl_ul_meas() from OsmoBTS
but optionally includes DTX information.

Change-Id: Ib37107bcc9909e5105ea711de42d3fb1db7e8d9e
Reviewed-on: https://gerrit.osmocom.org/44
Tested-by: Jenkins Builder
Reviewed-by: Harald Welte <laforge@gnumonks.org>
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 834e5d6..1e2d323 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -92,6 +92,8 @@
 gsm0808_create_sapi_reject;
 gsm0808_prepend_dtap_header;
 
+gsm0858_rsl_ul_meas_enc;
+
 gsm338_get_sms_alphabet;
 
 gsm340_gen_oa;