BTS_Tests.ttcn: mark f_unitdata_mo() as private

Change-Id: Idd31608cd91631a92ea9d73e1db508605d7314d4
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index c95f48a..ea75937 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -3746,7 +3746,7 @@
 }
 
 /* Send UI frame from MS and expect it to arrive as RLL UNITDATA IND on Abis */
-function f_unitdata_mo(
+private function f_unitdata_mo(
 	RslLinkId link_id,
 	octetstring l3,
 	boolean exp_sacch := true, /* Should tolerate SACCH messages? */