AppArmor: Allow access to mount related files

apparmor-session-lockdown-no-deny test returns the following events:
$ sudo journalctl -b -t audit -o cat | aa_log_extract_tokens.sh DENIED
====
profile:/usr/bin/prestwood
apparmor:DENIED
denied_mask:r
operation:open
name:/proc/905/mountinfo
requested_mask:r
====
profile:/usr/bin/prestwood
apparmor:DENIED
denied_mask:r
operation:open
name:/etc/fstab
requested_mask:r
====
profile:/usr/bin/prestwood
apparmor:DENIED
denied_mask:r
operation:open
name:/proc/905/mountinfo
requested_mask:r
====
profile:/usr/bin/prestwood
apparmor:DENIED
denied_mask:r
operation:open
name:/proc/905/mounts
requested_mask:r

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

debian/usr.bin.prestwood

@@ -28,4 +28,7 @@
   dbus (send, receive) bus=session peer=(label=/usr/lib/gvfs/gvfs*),
 
   /run/systemd/journal/{socket,stdout} rw,
+  /proc/@{pid}/mountinfo r,
+  /proc/@{pid}/mounts r,
+  /etc/fstab r,
 }