c++: Fix bootstrap

[Nathan Sidwell]

I'm not sure why this didn't pop up on the earlier bootstrap.  Anyway, 
fixed thusly.

I made the prefix for dumping a	binding-vector slightly	too small.
Fixed thusly.

	gcc/cp/
         * ptree.c (cxx_print_xnode): Increase binding-vector prefix size.

-- 
Nathan Sidwell
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