Adapt to coq/coq#15867 (stream in gramlib)

src/gappatac.c

@@ -15,6 +15,9 @@ open Tacmach
 #else
 open Tacmach.New
 #endif
+#if COQVERSION >= 81600
+open Gramlib
+#endif
 open Names
 open Coqlib
 open Libnames