[net][aix] Add missing return value in IOCountersWithContext()

Amends shirou#1797
Fixes shirou#1806

Author: Lomanic

net/net_aix_cgo.go

@@ -30,7 +30,7 @@ func IOCountersWithContext(ctx context.Context, pernic bool) ([]IOCountersStat,
 		iocounters = append(iocounters, n)
 	}
 	if pernic == false {
-		return getIOCountersAll(iocounters)
+		return getIOCountersAll(iocounters), nil
 	}
 	return iocounters, nil
 }

net/net_aix_nocgo.go

@@ -89,7 +89,7 @@ func IOCountersWithContext(ctx context.Context, pernic bool) ([]IOCountersStat,
 		return nil, err
 	}
 	if pernic == false {
-		return getIOCountersAll(iocounters)
+		return getIOCountersAll(iocounters), nil
 	}
 	return iocounters, nil
 }