gsm: Add a way to handle incoming CM Updates.

A CM Update would cause an unhandled exception and release
a call/SMS. Add a stub for the CM update.

src/GSMProcessor.st

@@ -40,6 +40,7 @@ OsmoGSM.BSSAPManagement extend [
             OsmoGSM.GSM0808Helper msgCipherModeCmpl -> #mapCipherModeCompl:.
             OsmoGSM.GSM0808Helper msgAssComplete -> #mapAssComplete:.
             OsmoGSM.GSM0808Helper msgAssFailure -> #mapAssFailure:.
+            OsmoGSM.GSM0808Helper msgCMUpdate -> #mapCMUpdate:.
             }) at: self data type ifPresent: [:sel |
             ^ aCon perform: sel with: self.
         ].
@@ -442,6 +443,10 @@ hosting various transactions and dispatching to them.'>
             self assignmentFailure.]]
     ]
 
+    mapCMUpdate: aData [
+        <category: 'map-classmark'>
+    ]
+
     assignmentSuccess [
         <category: 'audio-connect'>