logging: enable file/line info if --debug is set

This helps a lot to find out where the errors come from.

Before:
> level=warning msg="lstat /sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/test_hello: no such file or directory"

After:
> level=warning msg="lstat /sys/fs/cgroup/user.slice/user-1000.slice/user@1000.service/test_hello: no such file or directory" func=github.com/opencontainers/runc/libcontainer.destroy file="github.com/opencontainers/runc/libcontainer/state_linux.go:44"

Presumably this comes with an overhead, but I guess no one is using
--debug by default anyway.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

main.go

@@ -190,6 +190,7 @@ func createLogConfig(context *cli.Context) logs.Config {
 		LogLevel:    logrus.InfoLevel,
 		LogFilePath: logFilePath,
 		LogFormat:   context.GlobalString("log-format"),
+		LogCaller:   context.GlobalBool("debug"),
 	}
 	if context.GlobalBool("debug") {
 		config.LogLevel = logrus.DebugLevel