alloc: make allocate_alloc_state and clear_alloc_state more consistent

Since both functions are using the same data type, they should either both
refer to it as void *, or both use the real type (struct alloc_state *).
Opt for the latter.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alloc.c

@@ -36,7 +36,7 @@ struct alloc_state {
 	int slab_nr, slab_alloc;
 };
 
-void *allocate_alloc_state(void)
+struct alloc_state *allocate_alloc_state(void)
 {
 	return xcalloc(1, sizeof(struct alloc_state));
 }