fix symbolic link panic

[mikebrow]

avoids panic when running into a symlink by ignoring the symlink needed for
https://github.com/containerd/cri/blob/master/test/e2e

before:

ltag -t "../project/script/validate/template" --excludes "vendor" --check -v
panic: read test/e2e: is a directory

goroutine 1 [running]:
main.main()
	/home/mike/go/src/github.com/kunalkushwaha/ltag/main.go:99 +0xa95

after fix:

ltag -t "../project/script/validate/template" --excludes "vendor" --check -v
_output/containerd

See: https://golang.org/pkg/path/filepath/ "Walk does not follow symbolic links." possibly fixed in 1.14.3 golang/go@7d27e87

Signed-off-by: Mike Brown [email protected]

[mikebrow]

needed for merging containerd/cri into containerd.. so we don't have to skip test/ to run header check

containerd/cri#1479

[kunalkushwaha]

Thank you @mikebrow for PR.