Fix errors raised by missing-prototypes

The recent changes in the Linux kernel globally enforce function
prototype checks. This means that it's crucial for each function
to have a declaration to validate that both the caller and the
callee expect the same argument types. Failure to comply with this
requirement may result in real bugs due to mismatched prototypes.
Therefore, in this commit, we need to add function prototypes to
the header files or declare functions with static to ensure
compatibility with these changes.

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;