Add system_file_type

This file resides on /system, so must be annotated with the
system_file_type attribute.

Test: Policy compiles.
Change-Id: I52b77fd60060ca85562c8742e9efd129f6973a10

sepolicy/init_eth0.te

 type init_eth0, domain;
-type init_eth0_exec, exec_type, file_type;
+type init_eth0_exec, system_file_type, exec_type, file_type;
 
 init_daemon_domain(init_eth0)