Allow system_server to read tombstones.

Address denials such as:
 avc:  denied  { read } for  name="tombstones" dev="dm-0" ino=765537 scontext=u:r:system_server:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=dir
 avc:  denied  { open } for  name="tombstones" dev="dm-0" ino=765537 scontext=u:r:system_server:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=dir
 avc:  denied  { getattr } for  path="/data/tombstones/tombstone_00" dev="dm-0" ino=765538 scontext=u:r:system_server:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=file
 avc:  denied  { read } for  name="tombstone_00" dev="dm-0" ino=765538 scontext=u:r:system_server:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=file
 avc:  denied  { open } for  name="tombstone_00" dev="dm-0" ino=765538 scontext=u:r:system_server:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=file

Change-Id: Iae5a10bed9483589660b84a88b6b9f8f8e9a8f5c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>