kernfs: add struct dentry declaration in kernfs.h

Hello, Greg.

Two misc fixes for kernfs.

Thanks.
------- 8< -------
struct dentry is used in kernfs.h but its declaration was missing,
leading to compilation errors unless its declaration gets pulled in in
some other way.  Add the declaration.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernfs.h

@@ -18,6 +18,7 @@
 #include <linux/completion.h>
 
 struct file;
+struct dentry;
 struct iattr;
 struct seq_file;
 struct vm_area_struct;