bts/BTS_Tests: cosmetic: mark c_MA_null as private

Change-Id: I49d231395a6ae3a582c9719eaa5244daac32e020
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 7d211de..aefab09 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -3152,7 +3152,7 @@
 /***********************************************************************
  * Immediate Assignment / AGCH
  ***********************************************************************/
-const MobileAllocation c_MA_null := {
+private const MobileAllocation c_MA_null := {
 	len := 0,
 	ma := ''B
 }