Small cleanups to panic() prototype.

- Let panic() use _Noreturn and __printflike() instead of GCC-specific
  attributes.
- Remove prototype from ffs_subr.c and let it include ffs_extern.h.
- Forward declare struct inode to make ffs_extern.h not depend on it.

usr.sbin/makefs/ffs/ffs_extern.h

@@ -35,6 +35,8 @@
 
 #include "ffs/buf.h"
 
+struct inode;
+
 /*
  * Structure used to pass around logical block paths generated by
  * ufs_getlbns and used by truncate and bmap code.
@@ -46,8 +48,7 @@ struct indir {
 };
 
 	/* ffs.c */
-void panic(const char *, ...)
-    __attribute__((__noreturn__,__format__(__printf__,1,2)));  
+_Noreturn void panic(const char *, ...) __printflike(1, 2);
 
 	/* ffs_alloc.c */
 int ffs_alloc(struct inode *, daddr_t, daddr_t, int, daddr_t *);

usr.sbin/makefs/ffs/ffs_subr.c

@@ -38,11 +38,9 @@ __FBSDID("$FreeBSD$");
 
 #include <ufs/ufs/dinode.h>
 #include <ufs/ffs/fs.h>
+#include "ffs/ffs_extern.h"
 #include "ffs/ufs_bswap.h"
 
-void    panic __P((const char *, ...))
-    __attribute__((__noreturn__,__format__(__printf__,1,2)));
-
 /*
  * Update the frsum fields to reflect addition or deletion 
  * of some frags.