Fixed assertion in PyGccPrettyPrinter_as_string (gcc-python-pretty-printer.c)

Fixed assertion in PyGccPrettyPrinter_as_string (gcc-python-pretty-
printer.c), which was systematically failing;

This fix is a hack, just the bare minimum to get it to work.
Added the ability to access DECL_NONADDRESSABLE_P

For a FieldDecl, DECL_NONADDRESSABLE_P says whether the field is a
bitfield (at least in gcc version 4.8, as far as I could tell, the
nonaddressable property is a defacto equivalent to the field being a
bitfield).