MAC/do: Visually separate some file sections

With additional empty lines.

No functional change (intended).

Reviewed by:    bapt
MFC after:      1 month
Sponsored by:   The FreeBSD Foundation
Pull Request:   https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

sys/security/mac_do/mac_do.c

@@ -33,6 +33,7 @@
 
 #include <security/mac/mac_policy.h>
 
+
 #ifdef INVARIANTS
 /*
  * Should typically be moved to libkern (and perhaps libc) at some point, and be
@@ -226,6 +227,7 @@ struct id_elem {
 
 STAILQ_HEAD(id_list, id_elem);
 
+
 #ifdef INVARIANTS
 static void
 check_type(const id_type_t type)