coccinelle: misc: add flexible_array.cocci script

One-element and zero-length arrays are deprecated [1]. Kernel
code should always use "flexible array members" instead, except
for existing uapi definitions.

The script warns about one-element and zero-length arrays in structs.

[1] commit 68e4cd17 ("docs: deprecated.rst: Add zero-length and
    one-element arrays")

Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>