HSH_DerefObjCore signedness fix

noticed by Dridi here:
https://github.com/varnishcache/varnish-cache/pull/3434/commits/f9f40b991f301fb96d2e2d0d52b86c3d4b1f9f7f#r502302785

The reference count returned already is an unsigned value

bin/varnishd/cache/cache_hash.c

@@ -975,7 +975,7 @@ HSH_DerefBoc(struct worker *wrk, struct objcore *oc)
  * Returns zero if target was destroyed.
  */
 
-int
+unsigned
 HSH_DerefObjCore(struct worker *wrk, struct objcore **ocp, int rushmax)
 {
 	struct objcore *oc;

bin/varnishd/cache/cache_objhead.h

@@ -66,7 +66,7 @@ struct boc *HSH_RefBoc(const struct objcore *);
 void HSH_DerefBoc(struct worker *wrk, struct objcore *);
 void HSH_DeleteObjHead(const struct worker *, struct objhead *);
 
-int HSH_DerefObjCore(struct worker *, struct objcore **, int rushmax);
+unsigned HSH_DerefObjCore(struct worker *, struct objcore **, int rushmax);
 #define HSH_RUSH_POLICY -1
 
 enum lookup_e HSH_Lookup(struct req *, struct objcore **, struct objcore **);

bin/varnishd/cache/cache_req_body.c

@@ -259,7 +259,7 @@ VRB_Ignore(struct req *req)
 void
 VRB_Free(struct req *req)
 {
-	int r;
+	unsigned r;
 
 	CHECK_OBJ_NOTNULL(req, REQ_MAGIC);
 
@@ -268,7 +268,6 @@ VRB_Free(struct req *req)
 
 	r = HSH_DerefObjCore(req->wrk, &req->body_oc, 0);
 
-
 	// each busyobj may have gained a reference
 	assert (r <= req->restarts + 1);
 }