golint: fix warnings in daemon/logstash.go
golint: fix warnings in daemon/logstash.go

Fixes the following warnings

Line 28: warning: exported type LogstashStat should have comment or be unexported (golint)
Line 57: warning: exported method Daemon.EnableLogstash should have comment or be unexported (golint)

Related-to: #153 (Resolve golint warnings)
Signed-off-by: Alexander Alemayhu <e46fc836cca3acec03944314d1457c2ae6c68ef3@alemayhu.com>

cilium/cilium,mskarbek/cilium,eloycoto/cilium,mskarbek/cilium,gumpt/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,mskarbek/cilium,mskarbek/cilium,scanf/cilium,eloycoto/cilium,scanf/cilium,eloycoto/cilium,eloycoto/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,gumpt/cilium,gumpt/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,gumpt/cilium,mskarbek/cilium,tklauser/cilium,tgraf/cilium,scanf/cilium,cilium-team/cilium,tgraf/cilium,gumpt/cilium,tgraf/cilium,eloycoto/cilium,tklauser/cilium,cilium/cilium,mskarbek/cilium,cilium-team/cilium,eloycoto/cilium,scanf/cilium,scanf/cilium,scanf/cilium,gumpt/cilium,gumpt/cilium,scanf/cilium,michi-covalent/cilium,eloycoto/cilium