SELinux: inline selinux_is_enabled in !CONFIG_SECURITY_SELINUX

Without this patch building a kernel emits millions of warning like:

include/linux/selinux.h:92: warning: ?selinux_is_enabled? defined but not used

When it is build without CONFIG_SECURITY_SELINUX.  This is harmless, but
the function should be inlined, so it gets compiled out.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>