Add leading whitespace to DISTRO_FEATURES_append in oe-selinux.conf

The lack of leading whitespace was causing two values in the
DISTRO_FEATURES variable to be combined. This was causing 'sysvinit'
from DISTRO_FEATURES_BACKFILL and 'pam' (from oe-selinux) to be
combined into 'sysvinitpam' thus dropping both from the
DISTRO_FEATURES.

Signed-off-by: Philip Tricca <flihp@twobit.us>