Set kptr_restrict to 2.

To make writing kernel exploits harder, set /proc/sys/kernel/kptr_restrict
to "2". This prohibits users from accessing kernel symbols via /proc/kallsyms

Bug: 5555668
Change-Id: Ib31cb6fcb4d212a0b570ce9e73ae31f721ed801b

rootdir/init.rc

@@ -65,6 +65,7 @@ loglevel 3
     write /proc/sys/kernel/sched_compat_yield 1
     write /proc/sys/kernel/sched_child_runs_first 0
     write /proc/sys/kernel/randomize_va_space 2
+    write /proc/sys/kernel/kptr_restrict 2
 
 # Create cgroup mount points for process groups
     mkdir /dev/cpuctl