bugfix: fix initialUEMessage criticality

S1AP specs say:

    initialUEMessage S1AP-ELEMENTARY-PROCEDURE ::= {
        INITIATING MESSAGE      InitialUEMessage
        PROCEDURE CODE          id-initialUEMessage
        CRITICALITY             ignore
    }

So let's use "ignore", not "reject".

The online decoder http://asn1-playground.oss.com/ did
not decode a message encoded by the eNB because of this.

Only the S1AP messages captured while doing a simple connection
of one UE have been tested with the online decoder.

So, not all the S1AP messages have been checked, maybe some
instances of this problem are still there.

It does not seem to harm the actual UE<->eNB<->EPC communication
(because we don't check the integrity of messages or because
asn1c does not do it), so it's not a big deal.