Added Build Rule

Add build rule, to only build on Linux platform, reason for adding this: This allows the usage of the logrus package within a Windows based project. Current logrus project fails to import due to the fact the syslog is missing on Windows.

hooks/syslog/syslog.go

diff --git a/hooks/syslog/syslog.go b/hooks/syslog/syslog.go
index b6fa374..c59f331 100644
--- a/hooks/syslog/syslog.go
+++ b/hooks/syslog/syslog.go
@@ -1,3 +1,5 @@
+// +build !windows,!nacl,!plan9
+
 package logrus_syslog
 
 import (