Move mtectrl to private

Because mtectrl is a system internal domain, and we don't need to expose
the type to vendor.

Test: build and boot
Change-Id: Idb5c4a4c6f175e338722971944bf08ba99835476
diff --git a/private/mtectrl.te b/private/mtectrl.te
index a89edda..436dcae 100644
--- a/private/mtectrl.te
+++ b/private/mtectrl.te
@@ -1,4 +1,5 @@
 # mtectrl is a tool to request MTE (Memory Tagging Extensions) from the bootloader.
+type mtectrl, domain, coredomain;
 type mtectrl_exec, system_file_type, exec_type, file_type;
 
 init_daemon_domain(mtectrl)