Interworking: Fix network selection warning without SIM/USIM support

Interworking: Fix network selection warning without SIM/USIM support

interworking_credentials_available_3gpp() would have left excluded2
uninitialized without INTERWORKING_3GPP in the build. This could result
in a static analyzer warning within
interworking_credentials_available_helper() about use of uninitialized
variable. Get rid of that warning by explicitly initializing excluded2
even though this does not really result in any difference in behavior
since the excluded2 value would be used only if the non-NULL is returned
and that could not have been the case here without INTERWORKING_3GPP.

Signed-off-by: Jouni Malinen <j@w1.fi>