Use a forward definition of an opaque structure rather than a void

to avoid a strict alias type check failure in gcc 4.2.

include/dirent.h

@@ -60,6 +60,7 @@
 #define	DIRBLKSIZ	1024
 
 struct _telldir;		/* see telldir.h */
+struct pthread_mutex;
 
 /* structure describing an open directory. */
 typedef struct _dirdesc {
@@ -71,7 +72,7 @@ typedef struct _dirdesc {
 	long	dd_seek;	/* magic cookie returned by getdirentries */
 	long	dd_rewind;	/* magic cookie for rewinding */
 	int	dd_flags;	/* flags for readdir */
-	void	*dd_lock;	/* hack to avoid including <pthread.h> */
+	struct pthread_mutex	*dd_lock;	/* lock */
 	struct _telldir *dd_td;	/* telldir position recording */
 } DIR;