Fix errors raised by missing-prototypes

Fix compile time errors "no previous prototype for simplefs_mount"
and "no previous prototype for simplefs_kill_sb"

simplefs.h

@@ -108,12 +108,19 @@ struct simplefs_dir_block {
 
 /* superblock functions */
 int simplefs_fill_super(struct super_block *sb, void *data, int silent);
+void simplefs_kill_sb(struct super_block *sb);
 
 /* inode functions */
 int simplefs_init_inode_cache(void);
 void simplefs_destroy_inode_cache(void);
 struct inode *simplefs_iget(struct super_block *sb, unsigned long ino);
 
+/* dentry function */
+struct dentry *simplefs_mount(struct file_system_type *fs_type,
+                              int flags,
+                              const char *dev_name,
+                              void *data);
+
 /* file functions */
 extern const struct file_operations simplefs_file_ops;
 extern const struct file_operations simplefs_dir_ops;