CLEANUP: use struct comp_ctx instead of union

Replace union comp_ctx by struct comp_ctx.

Use struct comp_ctx * in the init/add_data/flush/reset/end prototypes of
compression.h functions.
diff --git a/src/proto_http.c b/src/proto_http.c
index cb24eb0..7f2c806 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2111,14 +2111,14 @@
 	}
 
 	/* initialize compression */
-	if (s->comp_algo->init(&s->comp_ctx.strm, 1) < 0)
+	if (s->comp_algo->init(&s->comp_ctx, 1) < 0)
 		goto fail;
 
 	return 1;
 
 fail:
 	if (s->comp_algo) {
-		s->comp_algo->end(&s->comp_ctx.strm);
+		s->comp_algo->end(&s->comp_ctx);
 		s->comp_algo = NULL;
 	}
 	return 0;