dir.h: remove duplicate forward declaration of struct repository

The `struct repository;` forward declaration appears twice in `dir.h`:
once at line 10 and again at line 46. This duplication is unnecessary
and likely unintentional.

Removing the second declaration has no impact on compilation, as verified
by a clean build.

Signed-off-by: Abhijeetsingh Meena <[email protected]>

Author: Ethan0456

dir.h

@@ -43,7 +43,6 @@ struct repository;
  *
  */
 
-struct repository;
 
 struct dir_entry {
 	unsigned int len;