fs/nullfs/null.h: put first definition after headers include

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D52983

Author: kostikbel

sys/fs/nullfs/null.h

@@ -35,11 +35,11 @@
 #ifndef	FS_NULL_H
 #define	FS_NULL_H
 
-#define	NULLM_CACHE	0x0001
-
 #include <sys/ck.h>
 #include <vm/uma.h>
 
+#define	NULLM_CACHE		0x0001
+
 struct null_mount {
 	struct mount	*nullm_vfs;
 	struct vnode	*nullm_lowerrootvp;	/* Ref to lower root vnode */