fix GH #161: add missing #include"containers.h"

parrot has now -Werror=missing-prototypes, which caught this omission

Author: Reini Urban

src/vm/parrot/6model/containers.c

@@ -2,6 +2,7 @@
 #include "parrot/parrot.h"
 #include "parrot/extend.h"
 #include "sixmodelobject.h"
+#include "containers.h"
 
 /* ***************************************************************************
  * CodePair container configuration: container with FETCH/STORE code refs