Fix tar command in ext Rakefile to use explicit -z flag.

This flag is necessary on some platforms.

Author: jemc

ext/rabbitmq/Rakefile

@@ -32,7 +32,7 @@ file_task 'rabbitmq-c.tar.gz', :download_tarball do
 end
 
 file_task 'rabbitmq-c', :download => :download_tarball do
-  cmd "tar -xf #{FILES[:download_tarball]}"
+  cmd "tar -zxf #{FILES[:download_tarball]}"
   cmd "mv rabbitmq-c-* #{FILES[:download]}"
 end