[CN] cannot mention extern arrays without size

[peterohanley]

% cn verify prepro.c
cn: internal error, uncaught exception:
    Failure("the concrete memory model cannot sizeof an array(_,none), from signed int[]")
    Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
    Called from Cerb_frontend__Impl_mem.Concrete.sizeof_ival in file "memory/concrete/impl_mem.ml", line 2465, characters 19-28
    Called from Cn__Memory.size_of_ctype in file "backend/cn/lib/memory.ml", line 58, characters 22-52
    Called from Cn__Resources.range_size in file "backend/cn/lib/resources.ml", line 33, characters 13-36
    Called from Cn__Resources.upper_bound in file "backend/cn/lib/resources.ml", line 37, characters 45-58
    Called from Cn__Check.record_globals.(fun) in file "backend/cn/lib/check.ml", line 2305, characters 24-58
    Called from Cn__Typing.bind in file "backend/cn/lib/typing.ml", line 49, characters 17-20
    Called from Cn__Typing.bind in file "backend/cn/lib/typing.ml", line 49, characters 17-20
    Called from Cn__Typing.bind in file "backend/cn/lib/typing.ml", line 49, characters 17-20
    Called from Cn__Typing.run_to_pause in file "backend/cn/lib/typing.ml", line 64, characters 8-21
    Called from Dune__exe__Main.with_well_formedness_check in file "backend/cn/bin/main.ml", line 165, characters 8-85
    Called from Dune__exe__Main.with_well_formedness_check in file "backend/cn/bin/main.ml", line 157, characters 6-442
    Re-raised at Dune__exe__Main.with_well_formedness_check in file "backend/cn/bin/main.ml", line 176, characters 4-69
    Called from Cmdliner_term.app.(fun) in file "cmdliner_term.ml", line 24, characters 19-24
    Called from Cmdliner_eval.run_parser in file "cmdliner_eval.ml", line 35, characters 37-44
% gcc -c prepro.c
% cat prepro.c
        extern int a[];

just int a[]; is OK.