MINOR: htx: Move the macro IS_HTX_STRM() in proto/stream.h

The macro IS_HTX_STRM() only relies on stream flags. So move it in
proto/stream.h.
diff --git a/include/proto/stream.h b/include/proto/stream.h
index 1461554..60338c9 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -31,6 +31,8 @@
 #include <proto/stick_table.h>
 #include <proto/task.h>
 
+#define IS_HTX_STRM(strm) ((strm)->flags & SF_HTX)
+
 extern struct pool_head *pool_head_stream;
 extern struct list streams;