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 <[e-mail]>
Reported-by: Randy Dunlap <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

security/smack/smackfs.c

···
 #include <linux/seq_file.h>
 #include <linux/ctype.h>
 #include <linux/audit.h>
+#include <linux/magic.h>
 #include "smack.h"
 
 /*