postinstall: allow twrp to run postinstall scripts

Change-Id: I9b2ff1b94611bf08e53a5cc452872eb3942dd160
diff --git a/sepolicy/twrp.te b/sepolicy/twrp.te
index 67eb563..6f43dc2 100755
--- a/sepolicy/twrp.te
+++ b/sepolicy/twrp.te
@@ -8,4 +8,5 @@
   allow kernel tmpfs:file { read };
   allow kernel recovery:fd { use };
   allow vendor_init rootfs:dir read;
+  allow postinstall tmpfs:file { getattr read execute };
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