Allow traced_probes to list the system partition

Relevant denies:

[    2.560660] type=1400 audit(1519404055.529:9): avc: denied { read }
for pid=896 comm=traced_probes name=system dev=sda22 ino=17
scontext=u:r:traced_probes:s0 tcontext=u:object_r:system_file:s0
tclass=dir permissive=0

Allowing only read then gives:
[    2.554718] type=1400 audit(1519404863.506:9): avc: denied { open }
for pid=890 comm="traced_probes" path="/system" dev="sda22" ino=17
scontext=u:r:traced_probes:s0 tcontext=u:object_r:system_file:s0
tclass=dir permissive=0

Test: flashed and ran directory listing code.
Bug: 73625480