GTP: convert from translation port to dual faced port

I spent numerous hours in trying to avoid the dual faced port from
segfaulting, but in the end didn't succeed :(

The easy way was then simply to fall back to the known-working pattern
of the dual-faced port, which I already successfully used also on the NS
module for the Gb Inteface
diff --git a/ggsn_tests/GTP_CodecPort_CtrlFunct.ttcn b/ggsn_tests/GTP_CodecPort_CtrlFunct.ttcn
index 33a2cab..4f61af7 100644
--- a/ggsn_tests/GTP_CodecPort_CtrlFunct.ttcn
+++ b/ggsn_tests/GTP_CodecPort_CtrlFunct.ttcn
@@ -40,4 +40,11 @@
     out UserData userData
   ) return Result;
 
+  external function f_GTPU_listen(
+    inout GTPU_PT portRef,
+    in HostName locName,
+    in PortNumber locPort,
+    in ProtoTuple proto,
+    in OptionList options := {}
+  ) return Result;
 }