Smack: include magic.h in smackfs.c

As reported for linux-next: Tree for Apr 2 (smack)
Add the required include for smackfs.c
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/smack/smackfs.c

@@ -26,6 +26,7 @@
 #include <linux/seq_file.h>
 #include <linux/ctype.h>
 #include <linux/audit.h>
+#include <linux/magic.h>
 #include "smack.h"
 
 /*