fix: import "math" in generated code for uint8 unmarshalling

[NexZhu]

This fix the following bug:

Currently generated code for unmarshalling uint8 field:

	// t.Type (service.TimeFilterType) (uint8)

	maj, extra, err = cbg.CborReadHeaderBuf(br, scratch)
	if err != nil {
		return err
	}
	if maj != cbg.MajUnsignedInt {
		return fmt.Errorf("wrong type for uint8 field")
	}
	if extra > math.MaxUint8 {
		return fmt.Errorf("integer in input was too large for uint8 field")
	}
	t.Type = TimeFilterType(extra)

math package is used in above code but the import statement for math is not generated, so generated code won't compile.

@Stebalien

[Stebalien]

Thanks!