Make ppp domain enforcing.

Change-Id: If6b85fbb2332f7a03b603f2d46bd2f73c778ecf9
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

ppp.te

 # Point to Point Protocol daemon
 type ppp, domain;
-permissive_or_unconfined(ppp)
 type ppp_device, dev_type;
 type ppp_exec, exec_type, file_type;
 domain_auto_trans(mtp, ppp_exec, ppp)