MEDIUM: checks/http-fetch: Support htx prefetch from a check for HTTP samples

Some HTTP sample fetches will be accessible from the context of a http-check
health check. Thus, the prefetch function responsible to return the HTX message
has been update to handle a check, in addition to a channel. Both cannot be used
at the same time. So there is no ambiguity.
diff --git a/include/proto/http_fetch.h b/include/proto/http_fetch.h
index 8162059..562e703 100644
--- a/include/proto/http_fetch.h
+++ b/include/proto/http_fetch.h
@@ -26,9 +26,10 @@
 #include <common/htx.h>
 #include <types/arg.h>
 #include <types/channel.h>
+#include <types/checks.h>
 #include <types/sample.h>
 
-struct htx *smp_prefetch_htx(struct sample *smp, struct channel *chn, int vol);
+struct htx *smp_prefetch_htx(struct sample *smp, struct channel *chn, struct check *check, int vol);
 int val_hdr(struct arg *arg, char **err_msg);