fix go vet issue
fix go vet issue

Prior to this commit, the test cases were failing on Go 1.10,
as the latest version of Go enables go vet by default when
running go test.

    ./types_test.go:41: Errorf call needs 1 arg but has 2 args
    FAIL	github.com/lunixbochs/struc [build failed]

lunixbochs/struc