CTRL: Fix tr_CtrlMsgGet containing invalid value in val field

"val" field is not used in GET commands and is hence set to omit by
TTCN3 decoder.

Change-Id: If1a273a2be71040eaea2189a0aeaf737adf848e5
diff --git a/library/Osmocom_CTRL_Types.ttcn b/library/Osmocom_CTRL_Types.ttcn
index 52be956..36e6762 100644
--- a/library/Osmocom_CTRL_Types.ttcn
+++ b/library/Osmocom_CTRL_Types.ttcn
@@ -124,7 +124,7 @@
 		verb := "GET",
 		id := id,
 		variable := variable,
-		val := ?
+		val := omit
 	}
 }