If the SELinux debug configuration is enabled define the macro DEBUG
such that pr_debug() calls are always enabled, regardless of
CONFIG_DYNAMIC_DEBUG, since those message are the main reason for this
configuration in the first place.

Mention example usage in case CONFIG_DYNAMIC_DEBUG is enabled in the
help section of the configuration.
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>