coccinelle: misc: restrict patch mode in flexible_array.cocci

Skip patches generation for structs with a single field.
Changing a zero-length array to a flexible array member in a struct
with no named members breaks the compilation. However, reporting
such cases is still valuable, e.g. commit 637464c5
("ACPI: NFIT: Fix flexible_array.cocci warnings").

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>