Add /system/bin/getprop as toolbox_exec

getprop is broken out from toolbox/toybox, however its permissions
should remain the same, so label it appropriately.

Bug: 36001741
Test: boot bullhead with the new getprop
Change-Id: I4114ea21998da95173d882038bc6aebf39b64d7f

private/file_contexts

@@ -187,6 +187,7 @@
 /system/bin/tune2fs	--	u:object_r:fsck_exec:s0
 /system/bin/toolbox	--	u:object_r:toolbox_exec:s0
 /system/bin/toybox	--	u:object_r:toolbox_exec:s0
+/system/bin/getprop		u:object_r:toolbox_exec:s0
 /system/bin/logcat	--	u:object_r:logcat_exec:s0
 /system/bin/logcatd	--	u:object_r:logcat_exec:s0
 /system/bin/sh		--	u:object_r:shell_exec:s0