fix logstreamer init file_match config

on git branch dev:
in plugins/logstreamer/logstreamer_input.go file, the Init function of LogstreamerInput, have some code like this:
if len(conf.FileMatch) > 0 && conf.FileMatch[len(conf.FileMatch)-2:] != "$" {
conf.FileMatch += "$"
}

but I think conf.FileMatch[len(conf.FileMatch)-2:] != "$" is not correct, because conf.FileMatch[len(conf.FileMatch)-2:] always return 2 length string, it will never equal to "$", so this condition is always true. And when the "file_match" config to 1 length string, it will cause error.