autofs: drop unnecessary extern in autofs_i.h

autofs4_kill_sb() doesn't need to be declared as extern, and no other
functions in .h are explicitly declared as extern.

Link: http://lkml.kernel.org/r/20160812024739.12352.99354.stgit@pluto.themaw.net
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/autofs_i.h

@@ -271,4 +271,4 @@ static inline void autofs4_del_expiring(struct dentry *dentry)
 	}
 }
 
-extern void autofs4_kill_sb(struct super_block *);
+void autofs4_kill_sb(struct super_block *);