fast-import: make code "-Wpointer-arith" clean

The dereference() function to peel a tree-ish and find the underlying
tree expects arithmetic to (void *) to work on byte addresses.  We
should be reading the text of objects through a char * anyway.

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

fast-import.c

@@ -2848,7 +2848,7 @@ static struct object_entry *dereference(struct object_entry *oe,
 					unsigned char sha1[20])
 {
 	unsigned long size;
-	void *buf = NULL;
+	char *buf = NULL;
 	if (!oe) {
 		enum object_type type = sha1_object_info(sha1, NULL);
 		if (type < 0)